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University  of  California. 


< ;  i  FT  i  >]••• 


IS 


LIBRARY 

UNIVERSITY  OF 

CALIFORNIA. 


INSTRUMENTS    AND    PUBLICATIONS 


OF   THE 


LIBRARY 

UNIVERSITY  OF 

CALIFORNIA. 


UNITED  STATES  NAVAL  OBSERVATORY. 


"  FOUNDED   A.  D.    1842. 

JOHN   TYLER, 

PRESIDENT  OF  THE  U.  S. 

ABEL   P.    UPSHUR, 

* 

SECRETARY  OF  THE  NAVY." 

[Inscription  on  t/te  Tablet  over  the  door  of  the  Obstrvntory  \ 


PUBLISHED  BY  AUTHORITY  OF  THE  HON.   SECRETARY  OF  Till-    XAVY. 


REAR-ADMIRAL   C.    H.    DAVIS,    SUPERINTENDENT. 


WASHINGTON 


1845-1876. 


/// 


TABLE    OF    CONTENTS. 


P««e. 

I.  Founding  of  the  Observatory. 3 

II.   Position  of  the  Observatory 6 

III.  Superintendent!  of  the  Observatory 7 

IV.  Publications  of  the  Observatory 8 

INSTRUMENTS  OF  THE  OBSERVATORY: 

i.  The  Mural  Circle 13 

ii.  The  Transit  Instrument 16 

iii.  The  Prime  Vertical  Transit 18 

iv.  The  9.6-inch  Equatorial 19 

v.  The  Transit  Circle 21 

vi.  The  XXVI-inch  Equatorial 26 

PLATES. 

General  view  of  the  Observatory Frontispiece. 

Plate     I.  The  Transit  Instrument  and  Mural  Circle to  face  page  13 

Plate    II.  The  9.6-inch"  Equatorial to  face  page  19 

Plate  III.  The  Transit  Circle to  face  page  21 

Plate  IV.  The  XXVI-inch  Equatorial,  (general  view) to  face  page  29 

Plate    V.  The  XXVI-inch  Equatorial,  (eye-end) to  face  page  33 

Plate  VI.  Astronomical  drawings to  follow  page  45 


I -FOUNDING  OF  THE  OBSERVATORY. 


[Condensed from  AniK-iiilix  IV  of  the  volume  of  Observations  for  1871:"   Founding  and  Progress  of  the  Olj.scrvatory," 

By  Prof.  |.  !•:.  XUURSE,  U.  S.  N.] 

Congressional  action  for  instituting  a  National  Observatory  originated  in  the 
earliest  movements  lor  establishing  a  first  meridian  in  the  United  States. 

Memorials  towards  tliis  object,  offered  by  Mr.  William  Lambert  of  Virginia,  were 
presented  iii  the  House  of  Representatives  as  early  as  the  years  iSi'o,  1815,  and  1818. 
They  were  approved  by  the  House,  and  Mr.  Lambert  was  appointed  to  make  astro- 
nomical observations,  in  order  to  ascertain  the  longitude  of  the  Capitol  from  Green- 
wich. In  his  re] >< >rts  < >f  these  observations,  transmitted  to  Congress  1  »\  1  'resident  Monroe 
in  1822  and  1823,  he  took  strong  grounds  in  advocacy  of  the  founding  of  an  Obsen- 
atory.1 

.Mr.  F.  1\.  Ilassler,  the  tirst  Superintendent  of  the  United  States  Coast  Survey,  in 
his  report  made  on  returning  from  the  purchase  of  his  instruments  in  London,  1816, 
recommended  the  establishment  of  an  Observatory  in  the  city  of  Washington  "as  a 
national  object,  a  scientific  ornament,  and  a  means  for  nourishing  an  interest  for  science 
in  general."2  Mr.  Hassler's  views  were  supported  by  many  eminent  men,  and  partic- 
ularly by  President  Madison  and  Secretary  Dallas,  who  were  desirous  that  this  part  of 
his  plans,  in  connection  with  the  work  of  the  Coast  Survey,  should  receive  immediate 
execution.  Mr.  Ilassler  submitted  a  detailed  plan  for  an  Observatory,  and  selected 
tor  it  a  site  north  of  the  Capitol. 

| 

In  1825  President  John  Qnincy  Adams,  in  his  first  message  to  Congress,  urged 
the  establishment  of  an  Astronomical  Institution,  referring  in  his  recommendation  to 
the  expectations  entertained  by  the  first  President  of  the  United  States,  and  his  selec- 
tion of  a  site  for  a  Scientific  Institution  at  the  Capital. 

In  1838  .Mi-.  Adams,  then  a  member  of  the  House  of  Representatives,  renewed 
his  efforts  for  the  object  by  urging  on  President  Van  Uuren  to  incorporate  the  idea  of 
an  ( )bservatory  within  the  plans  for  the  appropriation  of  the  Smithsonian  Fund.  In 
1842  he  made  an  extended  report  from  the  committee  appointed  upon  this  bequest, 
advocating  for  this  purpose  the  appropriation  of  a  large  part  of  the  interest  accruing 
from  the  fund. 

'Journal  of  the  House  of  Representatives,  Eighteenth  Congress,  FirM  Session  ;  Message  of  tin-  l'ri->idi-nt  of  Hie 
I'nited  States,  February  25,  1824. 

-  Transactions  of  American  Philosophical  Society  for  1825. 


4  FOUNDING  OF  THE  OBSERVATORY. 

Recommendations  for  the  founding  of  an  Observatory  had  also  been  made  by  the 
Secretary  of  the  Navy,  Mr.  Branch,  in  1830,  when  communicating  the  wishes  of  the 
Board  of  Navy  Commissioners,  and  by  his  successors,  Mr.  Dickerson,  in  1835,  and 
Mr.  Paulding,  in  1838. 

In  the  year  last  named,  a  series  of  astronomical  and  meteorological  observations 
was  commenced,  by  orders  of  the  Navy  Department,  in  the  small  Observatory  con- 
nected with  the  Depot  of  Charts,  under  charge  of  Lieut.  James  M.  Gilliss,  U.  S.  N., 
near  the  Capitol.  These  observations,  undertaken  in  connection  with  the  interests  of 
the  Exploring  Expedition  to  the  Southern  Hemisphere,  then  sailing  under  Captain 
Wilkes,  U.  S.  N.,  were  continued  to  the  year  1842.  They  brought  about  the  estab- 
lishment of  the  present  Institution.1 

The  Naval  Observatory  was  authorized  by  the  act  of  Congress  approved  by  the 
President  August  18,  1842.  The  full  purposes  of  this  act  were  declared  in  the  report 
of  the  committee  of  Congress2  recommending  the  appropriation,  which  report  was 
unanimously  accepted,  and  their  bill  passed  without  discussion.  In  this  report  astro- 
nomical work  in  its  full  scope  was  provided  for  in  express  terms. 

The  Secretary  of  the  Navy,  Hon.  A.  P.  Upshur,  directed  Lieut.  J.  M.  Gilliss,  U.  S.  X., 
who  had  been  in  charge  of  the  astronomical  and  Yneteorological  observations  befoie 
named,  to  prepare  a  plan  for  an  Observatory.  His  report,  presented  November  23, 
1843,  was  accepted  by  the  Department,  and  the  construction  of  a  building,  with  its 
equipments  for  astronomical  and  meteorological  work,  was  placed  in  his  charge.  The 
act  of  Congress  provided  for  its  erection  at  a  cost  not  exceeding  the  sum  of  $25,000. 

The  act  further  provided  that  the  institution  might  be  located  on  any  portion  of 
the  public  land  in  the  District  of  Columbia  which  the  President  of  the  United  States 
should  deem  suited  to  the  purpose.  On  the  recommendation  of  Lieutenant  Gilliss, 
reservation  No.  4,  as  marked  on  the  original  plan  of  Washington  City,  was  selected  by 
President  Tyler  as  the  site  of  the  Observatory.- 

This  reservation,  which  has  otherwise  a  marked  historic  interest,  had  long  been 
recognized  as  the  proposed  seat  of  a  scientific  institution.  For  this  pui'pose  it  was 
designated  by  General  Washington,  in  his  letter  of  October  21,  1796,  to  the  Commis- 
sioners appointed  to  lay  out  the  Capital.  It  had  been,  therefore,  known  in  Washington 
City  as  "University  Square." 

It  lies  on  the  north  bank  of  the  Potomac,  in  the  southwestern  section  of  the  city, 
the  north  front  on  E  street,  being  810  feet  in  length;  the  east,  on  Twenty-third  street, 
i,  103  feet ;  and  the  west,  on  Twenty-fifth  street,  620  feet.  The  area  inclosed  embraces 
a  little  more  than  nineteen  acres. 

"The  site  of  the  main  building  has  a  north  horizontal  range  of  1.25  miles,  and  a 
south  range  of  8  miles.  It  is  267  feet  from  the  north,  320  feet  from  the  east,  490  feet 
from  the  west,  and  900  feet  from  the  south  inclosure."  The  hill  is  of  gravel  formation, 
with  a  surface-stratum  of  dry,  brittle  clay. 

1  The  two  volumes  of  these  Observations,  published  by  order  of  Congress,  were  the  subjects  of  the  highest  com- 
mendation by  astronomers  in  Europe  as  well  as  in  the  United  States. 

"  Report  of  Naval  Committee,  House  of  Representatives,  No.  449,  Twenty-seventh  Congress,  Second  Session. 


KOIAIMM;  or  TIIK  OBSERVATORY.  5 

The  central  building  is  50  feet  8  inches  square  on  the  outside  from  the  founda- 
tion to  a  height  of  2  feet  6  inches  above  ground,  and  thence  to  the  top  of  the  walls,  50 
feet  square.  All  the  foundations  to  the  ground-line  are  of  lilne  rock,  2  feet  thick  ;  the 
remainder  of  the  outside  walls  are  of  brick,  18  inches  thick,  finished  in  the  best  man- 
ner; and  the  partition-vails  of  brick,  14  inches  thick.  It  is  two  stories  and  a  basement 
high,  with  a  parapet  and  balustrade  of  wood  around  the  top,  and  is  surmounted  bv  a 
revolving  dome,  23  feet  in  diameter,  resting  on  a  circular  wall  built  up  to  a  height  of 
7  feet  above  the  roof. 

To  the  east,  west,  and  south  sides  of  this  central  building  wings  were  built  bv 
Lieutenant  Gilliss.,  the  eastern  and  the  western  being  each  26  feet  in  length  and  21 
feet  in  width,  and  the  south  wing  being  21  feet  long,  with  the  same  breadth  and 
height  as  the  others. 

Additions  to  the  Observatory,  as  thus  originally  constructed,  have  been  made  at 
different  periods,  in  accordance  with  the  extension  of  its  astronomical  work.  In  the 
year  1.^47  quarters  for  the  Superintendent  were  erected  east  of  the  main  building.  In 
1848  the  east  wing  was  extended  24  feet,  connecting  these  quarters  with  the  main  build- 
ing and  furnishing  a  store-room  for  chronometers.  The  rating  and  daily  care  of  these, 
and  their  dispatch  to  United  States  vessels  when  put  in  commission,  are  intrusted  to 
the  officer  in  charge  of  this  room. 

Further  extensions  of  the  building  have  been  made  by  the  erection  of  the  ob.serv- 
ing-room  lor  the  Transit  Circle,  in  1868,  and  that  of  the  large  dome  for  the  26-inch 
Equatorial,  completed,  with  its  adjoining  offices  for  computers,  &c.,  in  1873. 

Each  of  these  latest  extensions  is  described  in  connection  with  its  instrument. 

"THE  LIBRARY. 

i 

During  the  official  visit  of  inquiry  made  by  Lieutenant  Gilliss,  in  1843,  to  the 
Observatories  in  Europe,  donations  of  nearly  three  hundred  Astronomical  and  other 
Scientific  volumes  were  offered  to  the  Observatory  then  being  founded  at  Washington. 

The  chief  donors  were,  the  Royal  Astronomical  Society,  the  Royal  Society,  the 
Admiralty,  the  East  India  Company,  and  the  Directors  of  the  Observatories  at  Green- 
wich, Berlin,  Brussels,  and  Munich.  More  than  seven  hundred  volumes  were  pur- 
chased by  Lieutenant  Gilliss. 

The  annual  Exchanges  of  Publications  made  by  the  Observatory  with  most  of 
the  scientific,  Institutions  of  the  world,  and  with  authors  of  distinguished  scientific 
treatises,  have  been  the  principal  sources  of  increase  up  to  the  present  date,  at  which 
the  numbers  in  the  Library  exceed  six  thousand.  These  exchanges  are  building  up  a 
scientific  Treasury,  largely  repaying  the  Government  for  its  outlay  in  the  publication 
of  the  Astronomical  volume*. 

In  transmitting  its  foreign  exchanges  the  Institution  is  indebted  for  the  courteous 
oflices  of  the  Smithsonian,  and  for  those  of  the  Resident  Legations. 


II -POSITION  OF  THE  OBSERVATORY.1 


The  latitude  deduced  from  observations  made  with  the  Mural  Circle  in  1 845  and 
1846  was,  +38°  53'  39".25;2  and  that  value  has  been  employed  in  the  reduction  of 
all  observations  made  with  the  Mural  Circle  down  to  the  present  time. 

The  observations  made  with  the  same  instrument  from  1 86 1  to  1 864,  inclusive, 
give  a  latitude  o".4/  less,  viz,  -(-38°  53'  38".8;3  and  this  latter  value  is  the  one  which 
has  always  been  used  in  computing  the  piiblished  "Apparent  North  Polar  Distances, 
obtained  from  Observations  made  with  the  Transit  Circle."  It  should  be  remarked, 
however,  that  in  the  pitblished  tables  of  "Right  Ascension,  North  Polar  Distances, 
and  Semi-diameters  of  the  Sun,  Moon,  and  Planets,  deduced  from  Observations  with  the 
Transit  Circle,"  and  of  "Corrections  to  the  Right  Ascensions  and  North  Polar  Dis- 
tances of  the  American  Kphemeris,  given  by  Individual  Observations  of  Stars  with  the 
Transit  Circle,"  the  final  north  polar  distances  depend  upon  the  position  of  the  pole 
determined,  from  all  the  observations  of  circumpolar  stars  made  with  the  Transit 
Circle  during  the  same  year  as  those  contained  in  the  tables  in  question.  In  interpo- 
lating tabular  positions  of  the  moon  and  planets,  the  longitude  of  the  Observatory  from 
Greemvich  is  assumed  to  be  5''  8m  1 2s.o. 

The  point  to  which  all  differences  of  longitude,  measured  from  the  Observatory, 
are  referred  is  the  center  of  the  dome.  The  most  probable  value  of  its  latitude  is 
+  38°  53'  38". 8,  which  has  been  determined  from  discussions  of  all  the  observations  of 
circumpolar  stars  observed,  both  above  and  below  the  pole,  with  the  Mural  and  Transit 
Circles  during  the  years  between  1861  and  the  present  time. 

For  the  determination  of  its  longitude  from  Greenwich  by  the  telegraph,  the 
following  data  were  communicated  in  an  official  letter  of  the  Superintendent  of  the 
Coast  Survey,  dated  August  10,  1872  : 

/*.  m.      a. 

Determined  in   1867  5  8   12.  11 

Determined  in  1870  12.16 

Determined  in   1872  12.10 


Mean  5  8   12.12 

1  The  astronomical  position  of  the  Observatory  here  given  appears  in  the  "  General  Introduction  "  to  each  of  the 
recent  volumes. 

-Washington  Observations  1X45,  Appendix,  page  116. 
•  Washington  Observations  1864,  Introduction,  page  44. 


Ill -SUPERINTENDENTS  OF  THE  OBSERVATORY. 


( 'ommander   M.    I''.    M  u  KY,  Iron i   (  Ictolier  i,   i  844,  to  April  20,  1861. 
(';i|)t;iiu  ,\.    !\l.   (Jiu.iss,  from  April  22,  i $6 1 ,  to  February  9,  1865. 
Kear-Adiiiiral  ( '.    II.    l>\vis,  from  April  28,  1865,  to  May  8,  1867. 
Rear-Admiral    !">.    F.   Svxns,  from  .May  8,  1867,  to  Fein-nary  23,  1874. 
liear-Admiral   ( '.    II.    MAVIS,  from  February  23,  1874, . 


OFFICERS  ix  CHARGE  OF  INSTRUMENTS  SINCE  1861. 


Tin-   9.6-hn-lt    J^iiiiituriii/.     Assistant  Astronomer  .  I.  Ferguson;   Professors  Ne\v- 
comli  ;    Hall  ;    Kastman. 

Tin-  I'l  inn-   ]'i  it'//  nl.  —  Professors  Hubltard;    Ncwcoin)). 

Tin'  Ti-in/xit  Iitstrutiii-itt.  —  Professor  Varrmll. 

Tin'  Mnnil  C'urli'.  —  Professors  Hitbbanl  ;    Xewcomh;    Vai'iiall. 

Tli/'  Tnnixit  ('/IT//:  —  Professors  Ncwcoinl)  ;   Hall;    Harkness;    Hastiuan. 

Tin'  \\\'l-iitili  .l-'.<i««tari<iL  —  Professors  Ne\vcoinli  ;    Hall. 

Tin'  Meteorological  Instruments.  —  Professor.!.  11.  Eastman. 


'/'///•  C/iriiittiiiti-fi'r*  inul  Xint/ird/  li<xtn(n;<'Htx.  —  Very  frequent  changes  have  occurred 
in  the  orders  of  the  officers  of  the  line  on  duty  at  the  Observatory  in  char-re  of  the 
naval  instruments  and  the  charts. 

At  the  date  of  the  or<rani/ation  of  the  HydrogTRphic  Office,  1866,  after  the  trans- 
fer of  the  charts  and  other  naval  instruments,  Commander  A.  \V.  Johnson  was  placed 
in  charge  of  the  <7//w>wr/ox  At  the  present  date  Lieutenant-Commander  Charles  II. 
1  >a\  is  is  in  charge. 


IV -PUBLICATIONS  OF  THE  OBSERVATORY. 


1.  VOLUMES  OF  OBSERVATIONS. 

Vol.    I.  Astronomical  Observations  for  die  year                   1845    published  in    1864 

II.                        Do.                         do.                             1  846             do. 

I85I 

III.                          Do.                           do.                                1847              do. 

1853 

IV.                          Do.                           do.                                1848              do. 

I856 

V.                           Do.                             do.                                  1849-1850  do. 

1859 

VI.                        Do.                         do.                             1851-1852  do. 

1867 

*VII.  Results  of  Astronomical  Observations  for  the  VISITS  1853-1860  do. 

1872 

VIII.  Astronomical  and  Meteorological  Observations  for  1861             do. 

1862 

IX.                        Do.                         do.                             1862             do. 

l863 

X.                          Do.                           do.                                1863              do. 

I865 

XI.                         Do.                          do.                               1864             do. 

1866 

XII.                        Do.                         do.                             1865             do. 

1867 

XIII.                        Do.                         do.                             1866             do. 

1868 

XIV.                        Do.                         do.                             1867             do. 

1870 

XV.                        Do.                         do.                             1868             do. 

1871 

XVI.                        Do.                         do.                             1869             do. 

1872 

XVII.                       Do.                        do.                            1870            do. 

'873 

XVIII.                        Do.                         do.                             1871             do. 

1873 

XIX.                         Do.                          do.                               1872              do. 

1874 

XX.                         Do.                          do.                               1873             do. 

1875 

XXI.                         Do.                          do.                               1874  in  pivss. 

•_>.  SPECIAL  OBSERVATIONS  AND  UKI'ORTS. 

Appendix  to 

(  Observation  <  >i   report. 

Author. 

volume  for 

the  year  — 

Suptrin/ciitltiiiv  of  Commander  Maurv. 

A.  —  Observations  'on  sun-spois  

Rev.  B.  Sestini,  S.  J  

B.  —  Observations  on  the  Mississippi    Kiver,  at  Memphis  

Passed  Midshipman  R.  A.  Marr  

j 

I                       Q        ^ 

C.  —  Tables  for  facilitating  reduction  of  apparent  places  of  fixed  stars  to 

Professors    [.   11.   (  '.   Coffin  and    I.   S. 

1847 

their  mean  places. 

Ilubbard. 

(/ones  of  stars  observed  al  ihe  National  (  Hiservalory  in  1846,  reduced 

Lieutenant  1..  Maynard  and    Professor 

J 

by  Assistant  Astronomer  James  Ferguson,  published  in  1860.) 

J.  H.  llnbbard. 

S:i!','i  :nl,  ii.l.'/i,  r  ../  Captain   Uilliss. 

Longitude   of  I'niled  Sl.ili  .    Naval  t  Hi  .<  i  \  al<»-y  from  moon  culmina- 

Professor S.  Ne  \vcom  b  

1862 

tions. 

A.—  Solar  parallax  from   ob-ei  \aiion-  on  the  planet   Mar^,  made    near 

Piofc-sor  A.  Hall  

~l 

,    the  opposition,  1862,  with  the  equatorial. 

A.  —  Solar  parallax   from  observation  ,  on   Hie   planet    Mars,  made  near 

\     i  Mni  A-tionoiner  T.  Ferguson  

1863 

the  opposition,  1862,  with  the  meridian-instrument. 

T'..      Orbit  of  Nemailsa      

Profe,soi    \     Hill 

hive  .libation  of  (he  latitude  and   longitude  of  (lie  (  Ibsei  \  alorv,  ami  of 

Profe   -or  •  S.  Ncucomb  

1864 

tin-  declination  of  certain  circunipolar  stars. 

'  This  \olume  \\as  published  as  Appendix  II   to  the  volume  of  (  Hiscrvalions  for  the  year  1871. 


R  Y 

PUBLICATIONS  OF  THE  OBSERVATORY.  g 

2.  SPECIAL  OBSERVATIONS  AND  REPORTS— Continued. 


Observation  or  report. 


Author. 


Appendix  to 
volume  for 
the  year — 


Supcrintonlcncy  of  Rear-Admiral  Davis. 

Description  of  the  Transit  Circle 

Investigation  of  the  distance  of  the  sun  . .. ... . 

Discussion  of  the  meteorological  observations  from  1842  to  1867 

Report  on  interoceanic  canals  and  railroads 

Report  on  November  meteors  of  1866 


Superintendency  of  Rear-Admiral  Sands. 

I. — Report  on  the  difference  of  longitude  between  Washington  and 
Havana. 

II. — Reports  on  the  total  solar  eclipse  in  the  United  States,  August  7, 
1869. 

III. — Positions  of  fundamental  stars  deduced  from  Washington  obser- 
vations, 1862- '67. 

IV — Catalogue  of  151  stars  in   Prresepe 

Catalogue  of  stars  observed  by  the  United  States  Astronomical  Expe- 
dition to  the  Sou' hern  Hemisphere,  l85O-'52. 


I. — Reports  on  total  solar  eclipse  in  Europe  of  December  22,  1870. . .. 

II. — Zones  of  stars  observed  with  the  Mural  Circle  in  the  years  1846, 
1847,  1848,  and  1849. 

I. — Report  on  the  difference  of  longitude  between  Washington  and 
Saint  Louis. 

II. — Reports  on  observations  of  Encke's  Comet,  during  its  return  in 
1871. 

III. — Right  ascensions  of  the  equatorial  fundamental  stars,  and  correc- 
tions necessary  for  reduction  of  right  ascensions  of  different  cata- 
logues to  a  mean  homogeneous  system. 

IV. — Zones  of  Stars  observed  at  the  United  States  Naval  Observatory 
with  the  Meridian  Transit  Instrument,  in  years  1846,  1847,  1848, 
and  1849. 

Report  on  November  meteors  of  1867 


Discussions  of  West  India  Cyclone  of  October  29  and  30,  1867 ( 

Report  on  November  meteors  of  1868 j 

I. — Zones  of  stars  observed  at  the  United  States  Naval  Observatory 
with  the  Meridian  Circle,  in  the  years  1847,  1848,  and  1849. 
[This  Appendix,  edited  by  Professor  A.  Hall,  I'.  S.  N  ,  completes 
the  Zone  Observations  commenced  in  1847,  and  embraces-  those  pub- 
lished in  1860,  1869,  1870,  and  1871 ;  those  published  in  1869,  1870, 
1871  were  reduced  by  Dr.  B.  A.  Gould.] 

II. — Results  of  observations  made  at  the  United  States  Naval  Observ- 
atory with  the  Transit  Instrument  and  Mural  Circle,  in  the  years 
1853  to  1860,  inclusive. 

III. — Catalogue  of  stars  observed  at  the  United  States  Naval  Observ- 
atory during  the  years  1845  to  1871. 
IV. — Memoir  of  the  founding  and  progress  of  the  United  States  Naval 

Observatory. 
I. — Tables  of  instrumental  constants  and  corrections  for  the  reduction  "\ 

of  Transit  observations. 

II. — Difference  of  longitude  between  Washington  and  Detroit,  Mich. ;  [ 
Carlin,  Nevada,  and  Austin,  Nevada.  J 

Superintendency  of  Rear- Admiral  Davis. 

I. — The  Uranian  and  Neptunian  systems  investigated  with  the  26-inch 

Equatorial. 
I. — The  Instruments  and  the  Publications  of  the  United  States  Naval 

Observatory. 
II. — Report  on  the  difference  of  longitude  between  Washington  and 

Ogden,  Utah. 
III. — Catalogue  of  Double  Stars 


Professor  S.  Newcomb .  . 


Professor  J.  R.  Eastman.   . 
Rear-Admiral  C.  H.  Davis . 


Professor  W.  Harkness. 


Professors  S.  Newcomb,  A.  Hall,  W. 

Harkness,  J.  R.  Eastman,  and  others. 

Professor  S.  Newcomb 


Professor  A.  Hall 

Lieutenant  James  M.  Gilliss,  Superin- 
tendent ;  Lieutenant  A.  McRae, 
Master  S.  Ledyard  Phelps,  and  Mr. 
E.  R.  Smith,  assistants. 

Professors  Newcomb,  Hall,  Harkness, 
and  Eastman. 

Professor  J.  H.  C.  Coffin,  Lieutenant 
T.  J.  Page,  Lieutenant  C.  Steedman. 

Professor  W.  Harkness 


Professors  Hall  and  Harkness . 
Professor  S.  Newcomb 


Lieutenant  J.  J.  Almy,  Lieutenant  W. 

A.  Parker,    Professors    R.    Keith, 
Mark  H.  Beecher,  J.  S.  Hubbard. 

Professors  Newcomb,  Harkness,  and 
Eastman. 

Professor  J.  R.  Eastman , 

Observers:    Professor  J.  Major,  Lieu- 
tenant L.  Maynard,  Lieutenant  W. 

B.  Muse. 


Professor  M.  YarnaH. 


Professor  M.  Yarnall ... 
Professor  J.  E.  Nourse  . 


Professor  J.  R.  Eastman. 


Professor  S.  Newcomb 


1865 
1866 


Professors  M.  Yarnall,  J.  R.  Eastman, 

and  E.  S.  1 1  olden. 
Professor  J.  R.  Eastman 


S.  W.  Burnham. . 


1867 


1868 


1869 


1870 


1871 


1872 

1873 
1874 


-74  APP  I 


THE   INSTRUMENTS 


OF  THE 


O  B  S  E  R  VAT  O  R  Y 


I. — The  MURAL  CIRCLE  Mounted  in  1 844. 

II. — The  TRANSIT  INSTRUMENT  do  1844. 

III. — The  PRIME- VERTICAL  TRANSIT-INSTRUMENT  do  1845. 

IV. — The  9.6-iNCH  EQUATORIAL  do  1845. 

V. — The  TRANSIT  CIRCLE  do  1865. 

VI. — The  XXVI-INCH  EQUATORIAL  -  do  1873. 


LIBRARY 

NIVERSITY  OF 

CALIFORNIA. 


Plate  I. 


Helioiype. 


TRANSIT   INSTRUMENT. 

U.  S.  NAVAL  OBSERVATORY,  WASHINGTON. 
MOUNTED  1845. 

Focal  Length,  7  ft.  0.4  in. 


OBJECT  GLASS 


K.  (  j^-oud  &  C'n  ,  Hoston. 


MURAL   CIRCLE. 
U.  S.  NAVAL  OBSERVATORY,  WASHINGTON. 
MOUNTED  1845. 

(  Focal  Length,  5  ft.  3.8  in. 


Clear  Aperture,  5.33  in. 


OBJECT  GLASS 


(Clear  Aperture,  4.10  in. 


I -THE  MURAL  CIRCLE. 


[Prepared  by  Professor  M.  YARNALL,  U.  S.  N.,  in  charge.] 


The  Mural  Circle  was  constructed  by  Troughton  &  Simms,  of  London.  It  was 
mounted  August  12,  1844,  in  the  room  which  is  now  used  as  the  Library,  but  was 
removed  during  the  latter  part  of  1 845  to  the  Observing  Room  in  the  east  wing  of  the 
building,  and  there  mounted  on  the  eastern  face  of  a  sandstone  pier.  This  pier  is 
9  feet  4  inches  high,  6  feet  from  north  to  south,  and  3  feet  3  inches  from  east  to  west. 

The  axis  of  the  instrument  reaches  entirely  across  the  pier,  through  an  archway 
of  32  inches  span  and  21  inches  pitch. 

The  Circle  is  placed  upon  the  eastern  end  of  the  axis,  which  is  sustained  in  part 
on  friction-rollers,  supported  by  counterpoises.  It  is  5  feet  in  diameter.  The  divisions 
are  upon  the  periphery  of  the  rim,  cut  upon  a  band  of  gold,  and  5'  apart.  They  are 
read  off  by  means  of  six  of  "Troughton's  Reading  Microscopes,"  mounted  firmly  upon 
the  face  of  the  pier,  and  adjusted,  as  nearly  as  practicable,  60°  from  each  other. 
These  microscopes  are  designated  by  the  letters  A,  B,  C,  D,  E,  F  ;  A  being  on  the  north 
side  of  the  pier,  and  the  line  through  the  center,  joining  A  an,d  B,  being  horizontal. 

By  this  arrangement — 

(The  reading  for)  B  —  (the  reading  for)  A  +  1 80° 

Do.  C=             do.              A +  300° 

Do.  D  =            do.             A  +  1 20° 

Do.  E=            do.             A +  240° 

Do.  F  =            do.             A+    60° 
approximative^. 

These  microscopes  are  adjusted,  as  to  focal  length,  so  that  five  revolutions  of  the 
micrometer-screw  of  each  may  measure  one  space,  or  5'  of  the  circle.  The  microm- 
eter-heads are  divided  into  sixty  parts,  reading,  therefore,  directly  to  seconds,  or,  by 
subdivisions  easily  made,  to  tentlis  of  seconds.  In  general,  in  making  iv:iding8  of  the 
Circle,  the  seconds  and  parts  are  read  from  each  of  the  six  microscopes,  and  to  these 
are  prefixed  the  degrees  and  m unites  from  A  alone.  The  mean  of  these  six  readings 
constitutes  the  "Circle  Reading." 

For  illuminating  the  graduated  limb,  an  argand  gas-burner  is  placed  behind  the 
pier,  in  a  line  with  the  axis  of  the  instrument.  The  light  from  this  burner  passes 
through  six  holes,  bored  through  the  pier,  directly  to  the,  reflectors  attached  to  the 
several  microscopes.  In  this  way  a  less  variable  illumination  is  secured  than  by  means 
of  lamps  earned  in  the  hand. 


Ill -THE  PRIME-VERTICAL  TRANSIT  INSTRUMENT. 


This  instrument  being  for  the  present  out  of  use,  no  detailed  description  of  it  is 
required,  further  than  that  given  in  Wdxltiityfoii  Astronomical  Observations,  vol.  i,  1845, 
page  li  et  seq.,  and  Plate  IV. 

Its  objective  has  an  aperture  of  4.86  inches,  and  a  focal  length  of  6  feet  5  inches. 
Its  four  eye-pieces  magnify — 

No.   i  74  diameters. 

Xo.   2  1 06  diameters. 

No.  3  140  diameters. 

No.  4  221   diameters. 

The  observations  made  with  this  instrument  may  be  found  in  the  annual  vol- 
umes. 


"X 


LIBRARY 

UNIVERSITY  OF 

CALIFORNIA. 


Plate  II. 


Heliotype. 


James  R.  Osgood  &  Co.,  Boston. 


THE    9.6     INCH     EQUATORIAL. 

U.  S.    NAVAL    OBSERVATORY,   WASHINGTON.         MOUNTED    1844. 


I.I  ii  K  A  K  i 

!    X  I  V  KIJ^ITV    or 

<  AUii'OKNIA. 


IV  -THE  9.G-INCH  EQUATORIAL 


[  I'rc|»nml  liy  I'rofessor  J.  R.  EASTMAN,  in  charge,  1876.] 


This  instrument  was  made  hv  Merz  &  Mahler,  of  Munich.  Its  object-glass  has 
a  clear  aperture  of  9.62  inches,  and  a  focal  length  of  14  feet  4.5  inches.  It  is  mounted 
in  the  (uTinan  stvle,  and  is  provided  with  a  finder,  hour  and  declination  circles,  a, 
micrometer,  with  a  position-circle,  and  a  driving-clock. 

The  finder  has  an  object-glass  2.6  inches  in  diameter,  with  a  focal  length  of  32 
inches. 

The  hour-circle  is  15  inches  in  diameter,  is  divided  to  single  minutes,  and  may  be 
read  by  means  of  a  vernier  to  2s. 

The  declination-circle  is  21  inches  in  diameter,  is  divided  to  5  minutes  of  arc,  and 
may  be  read  by  means  of  a  vernier  to  4". 

The  micrometer  is  provided  with  three  Ji.ml  transit-wires,  and  a  vioi'ttbh'  declina- 
tion sv.-tem  consisting  of  live  wires.  The  transit-wire  nearest  to  the  comb  of  the  microm- 
eter is  called  wire  A,  and  the  others  are  known  as  B  and  (J.  The  equatorial  interval 
of  B— A  is  i2".O3,  and  of  C— B  is  1 18.98. 

The  approximate  intervals  of  the  declination  system  in  micrometer-revolutions 
are,  numbering  the  wires  from  the  head  of  the  micrometer: 

1  to  2  1 6.86  revolutions. 

2  to  3  13.06  revolutions. 

3  to  4  13.10  revolutions. 

4  to  5  16.06  revolutions. 

The  value  of  one  revolution  of  the  micrometer-screw  \\as  found  in   1872  to  bo 

The  posifion-eircle  is  divided  to  15',  and  mav  be  read  hv  a  vernier  to  i'. 

The  illumination  of  the  field  or  of  the  wires  is  accomplished  by  directing  the  light 
from  a  gas-burner  through  openings  in  the  side  of  the  tube  and  toward  the  eye-piece, 
in  the  usual  manner. 

The  clock-work  for  moving  the  instrument  in  right  ascension  is  regulated  bv  a 
Frannhofer  centrifugal  pendulum,  but  it  is  scarcelv  powerful  enough  to  properly  con- 
trol the  motion. 

This  instrument  is  provided  with  eight  eye-pieces,  magnifying,  respecively,  90, 132, 
209,  296,  433,  562,  734,  and  899  diameters.  One  eye-piece  i<  fitted  with  a  Dingle-ring 
micrometer  and  one  with  a  double-ring'. 


2O  INSTRUMENTS  OF  THE  OBSERVATORY. 

There  is  a  chronograph  mounted  on  a  shelf  on  the  southwest  side  of  the  obaerving- 
room,  and  protected  by  doors  which  completely  inclose  it  when  not  in  use.  Jt  was 
made  by  Bond,  of  Boston,  and  is  regulated  by  "Bond's  spring  governor."  The  barrel 
is  6  inches  in  diameter,  13.5  inches  long,  revolves  once  in  a  minute,  and  the  record  of 
both  clock  and  key  is  effected  by  a  single,  glass,  fountain-pen.  This  chronograph  can 
be  connected  with  any  clock  which  it  is  desirable  to  use. 

THE  COMET  SEEKER. 

This  instrument  is  placed  on  a  stand  in  the  equatorial- room  when  not  in  use.  It 
was  made  by  Utzschneider  &  Frauenhofer,  is  equatorially  mounted,  and  has  an  object- 
glass  of  3.96  inches  diameter  and  32.4  inches  focal  length.  It  lias  an  hour-circ.e  5  inches 
in  diameter,  divided  on  silver,  and  reading  by  two  verniers  to  4",  and  a  similar  circle 
on  the  declination-axis  which  reads  by  verniers  to  i'.  The  instrument  has  five  eve- 
pieces,  whose  approximate  magnifying  powers  are,  respectively,  13.6,  19.4,  40.6,  40.8, 
and  41.6  diameters.  No.  5  is  furnished  with  a  ring  micrometer.  When  this  instrument 
is  to  be  used  it. is  placed  on  supports  outside  the  dome  on  the  roof. 

THE  9.6-INCH  EQUATORIAL  OBSERVING-ROOM 

The  Equatorial  is  supported  on  a  pier  whose  foundation  is  9  feet  below  the  sur- 
face of  the  ground.  The  diameter  of  the  foundation  is  15  feet.  It  is  built  up  solid  of 
stone  laid  in  hydraulic  cement  to  a  height  of  10.5  feet,  where;  the  diameter  is  12  feet. 
On  that  foundation  a  conical  pier  28  feet'  high  and  7  feet  in  diameter  at  the  top  is 
built.  The  walls  are  3  feet  thick  to  within  10  feet  of  the  top,  where  they  gradually 
increase  in  thickness,  and  the  last  3  feet  is  solid.  The  pier  is  capped  Avith  stone,  on 
which  rests  the  block  of  Maryland  granite  which  supports  the  instrument.  The  room 
is  circular  and  20.1  feet  in  diameter,  with  walls  7  feet  high.  The  hemispherical  dome 
revolves  on  six  equidistant,  iron,  32-pound  balls,  moving  in  the  groove  of  a  cast-iron  rail 
fastened  on  the  top  of  the  circular  wall.  The  frame-work  of  the  dome  is  of  wood, 
covered  with  half-inch  boards  sheathed  with  copper.  It  is  lined  inside  with  painted 
canvass.  The  observing-slit  is  20  inches  wide,  extends  from  2  feet  above  the  base  of 
the  dome  to  2  feet  beyond  the  center.  The  slit  is  covered  by  five  shutters,  each  open- 
ing from  the  inside  by  means  of  a  lever  and  pulley,  the  one  at  the  top  of  the  dome 
opening  first.  The  dome  is  turned  by  means  of  a  rack  and  pinion,  operated  by  a 
vertical  wheel  3  feet  in  diameter  and  fitted  with  projecting  handles. 

The  room  is  furnished  with  an  observing-chair  of  an  obsolete  pattern,  which  may 
be  moved  around  the  pier  on  a  circular  track. 


LIBRARY  N 

UNIVERSITY  OF 

CALIFORNIA. 


Plate  III. 


Heliotype. 


lames  R.  Osgood  &  Co.,  Boston. 


TRANSIT     CIRCLE. 
U.  S.   NAVAL   OBSERVATORY,   WASHINGTON.        MOUNTED    1866. 


'•:  OF 


V.-THE  TRANSIT  CIRCLE. 


:v<l  liy  I'rofi-sMir  J.  R.  EASTMAN,  in  charge,  |S76.] 


'I'liis  instrument  was  made  by  Pistor  &  Martins,  of  Hurl  in,  in  1865.  It  is  of  the 
reversible  pattern,  and  is  mounted  upon  t\vo  massive  marble  piers,  with  its  axis  S  feet 
2  inches  above  the  floor. 

The  telescope  has  a  clear  aperture  of  8.52  inches  and  a  local  length  of  12  feet 
and  0.7  inches.  The  axis  is  cast  in  a  single  piece,  into  which  the  steel  pivots,  2.09 
inches  in  diameter  and  1.70  inches  long,  are  screwed. 

The  Ys  are  of  gun-metal,  and  the  bearing-surfaces  are  0.28  of  an  inch  wide.  The 
distance  between  them,  measured  from  center  to  center  of  the  hearing-surfaces,  is  3  feet 
9.1  inches.  The  teleseope-tuhe  is  made  in  two  similar  parts,  which  are  bolted  to  the 
opposite  sides  of  the  cube  which  forms  the  central  portion  of  the  axis.  This  cube 
measures  16.64  inches  on  each  edge. 

Attached  to  the  two  extremities  of  the  axis  are  two  circles  identical  in  form  and 
si/.e.  Each  has  ten  radial  arms,  is  cast  in  a  single  piece  weighing  about  eighty  pounds, 
is  45.30  inches  in  'diameter  at  its  outside  edge  and  43.40  inches  at  the  graduation.  The 
circle  on  the  clamp-end  of  the  axis,  designated  as  circle  A,  has  inlaid  upon  its  face  two 
bands  of  silver,  each  0.130!' an  inch  wide, the  inner  one  of  which  is  graduated  to  every 
2'  and  the  outer  one  to  every  10'.  The  other,  designated  as  circle  l>,  has  inlaid  upon 
its  face  a  single  band  of  silver  0.13  of  an  inch  wide,  which  is  graduated  to  every  2' '. 

The  graduation  on  each  of  these  circles  is  numbered  from  o°  to  360°,  the  reading 
increasing  from  left  to  right,  and,  as  they  face  in  opposite  directions,  when  the  tele- 
scope is  moved  in  xenith-distance  the  reading  of  one  circle  increases  while  that  of  the 
other  diminishes. 

The  circles  are  attached  to  the  axis  in  such  a  manner  that  they  may  beset  Jo  give 
any  desired  reading  \\hen  the  telescope  is  pointed  to  the  zenith. 

Kach  pier  carries  four  microscopes,  placed  at  the  extremities  of  two  diameters, 
which  intersect  each  other  at  right  angles,  and  each  of  which  makes  an  angle  of  45° 
with  the  vertical.  They  are  attached  by  means  of  metal  arms,  covered  with  wood,  to 
the  brass  disk  on  the  face  of  the  pier  which  supports  the  Ys. 

The  microscopes  on  the  wotern  pier  are  marked,  I,  II,  HI,  IV.  and  those  on  the 
ea.-tern  pier,  V,  \  1.  yil,  \  III.  The  readings  of'  the  former  diminish,  and  those  of  the 
latter  increase,  as  the  telescope  moves  from  the  zenith  toward  the  south.  These  micro- 
scopes magnify  about  forty-five  diameters.  Kach  revolution  of  their  MMV\\S  is  equal 
to  30",  and  their  micrometer-heads  are  divided  to  o"-5.  Kach  microscope  micrometer 
is  furnished  with  two  parallel  threads  about  12"  apart,  and  the  reading  is  made  when 


s. 

a. 

c, 

4.1 

D, 

8.2 

C2 

2.O 

1)., 

9-7 

C3 

o.o 

V 

—  1)., 

12.  2 

c, 

2.O 

VI 

24-5 

05 

4.1 

VII 

36.8 

22  INSTRUMENTS  OF  THE  OBSERVATORY. 

the  image  of  the  division  on  the  limb  of  the  circle  is  exactly  midway  between  the 
threads.  In  addition  to  the  microscopes  already  mentioned  each  pier  carries  another, 
which  is  employed  as  a  pointer  for  setting  the  telescope  by  means  of  the  course-grad- 
uation on  circle  A. 

These  microscopes  magnify  twenty-three  diameters,  and  are  placed  at  the  extrem- 
ities of  horizontal  radii  to  the  circles,  that  on  the  western  pier  being  to  the  north  and 
that  on  the  eastern  pier  being  to  the  south  of  the  axis.  The  setting-microscope  in  actual 
use  is  always  the  one  at  the  clamp-end  of  the  axis. 

In  the  eye-piece  of  the  telescope  is  a  fixed  reticule  of  fifteen  vertical  and  two 
horizontal  wires.  The  latter  have  a  space  of  about  8"  between  them,  and  are  used 
principally  to  mark  the  center  of  the  field  of  view.  The  notation  and  the  approximate 
equatorial  intervals  between  each  vertical  wire,  and  the  mean  of  sets  B  and  ]),  a  (  „ 

C3,  and  C4  are  as  follows  : 

s. 
I  36.8 

n          24.5 

111=]*!        12.2     IV 

B,       9-7 

B3       8.2 

The  wires  Bj  to  B3  are  kno\v:n  as  set  B ;  Cl  to  C5,  set  C ;  and  Dj  to  D3,  set  D.  The 
wire  on  the  western  side  of  the  field  is  always  designated  as  wire  I;  consequently,  when 
the  instrument  is  reversed  the  notation  of  the  wires  is  reversed. 

In  addition  to  the  fixed  reticule  the  eye-piece  contains  a  right-ascension  and  a 
zenith-distance  micrometer,  in  each  of  which  the  readings  increase  as  the  wire  moves 
from  the  micrometer-head.  The  right-ascension  micrometer  is  used  in  determining 
the  collimation  constant, .and  sometimes  in  obtaining  the  time  of  transit  of  close 
circumpolar  stars  when  clouds  prevent  their  being  seen  at  the  fixed  wires.  It  is 
provided  with  a  single  vertical  wire  moved  by  a  micrometer-screw,  each  revolution 
of  which  measures  i".O24.  The  zenith-distance  micrometer  carries  four  horizontal 
wires,  the  middle  two  of  which  are  about  3"  apart,  and  the  center  of  the  space 
between  them  is  regarded  as  the  standard  middle  wire.  One  revolution  of  the  zenith- 
distance  micrometer  =  15". 3 12.  The  other  two  are  placed  at  a  distance  of  about  ten 
revolutions  of  the  screw  on  either  side  of  the  middle  wires,  that  nearest  the  head  of  the 
micrometer  being  called  wire  A  and  the  other  wire  B.  In  observing  southern  stars  the 
micrometer-head  is  below  the  eye-piece  if  the  clamp  is  west,  and  above  it  when  the 
clamp  is  east.  An  increase  of  micrometer  reading  is  subtractive  from  the  reading  of 
circle  A  and  additive  to  that  of  circle  B. 

The  illumination  of  the  field  of  view  is  so  arranged  that  the  wires  can  be  shown 
either  dark  on  a  bright  field  or  bright  on  a  dark  field.  In  the  case  of  bright  field- 
illumination  the  color  of  the  light  can  be  varied  from  deep  red,  through  light  red,  yel- 
low, and  light  blue,  to  dark  blue. 

The  instrument  is  provided  with  five  positive  eye-pieces,  magnifying,  respectively, 
135,  158,  1 86,  279,  and  395  diameters.  That  magnifying  186  diameters  is  generally 
used. 


INSTKI  MI;NTS  OF  •rni:  OBSKKVA  K>KY.  2-5 

COLLIMATOB8. 

The  'IV;iiisil  ( 'ircle  is  furnished  with  two  collini;itoi-s,  having  object  gla>ses  of  35. 2 
inches  focal  length  and  2.13  inches  clear  aperture.  The  eye-pieces  have  a  power  of  67. 
The  colliinators  an-  mounted  on  sandstone,  piers,  the  one  north  and  the  other  south 
of  tin-  Transit  Circle,  14.8  feet  from  the  center  of  the  instrument.  The  field  of  view 
of  collimator  A  contains  a  single  vertical  wire,  crossed  at  its  middle  point  by  a  hori- 
zontal wire.  The  lield  of  view  of  collimator  H  contains  a  fixed  wire  and  two  inter- 
secting wires,  movable  by  means  of  a  micrometer-screw.  Each  of  these  wires  makes 
an  angle  of  12°  with  the  vertical.  Collimator  A  is  usually  mounted  on  the  south  pier. 
Two  levels  are  provided  by  which  the  colliinators  may  be  placed  horizontal. 

Kach  side  of  the  central  cube  of  the  Transit  Circle  telescope  is  pierced  with  a 
circular  opening  2.3  inches  in  diameter,  which  is  closed  by  screwing  a  cap  into  it.  By 
removing  the  caps  and  setting  the  telescope  vertical  the  wires  of  either  collimator  may 
be  >cc]\  from  the  other. 

TIIK  STANDARD  S1PKKKAL  CLOCK. 

This  clock  is  Kesscls,  No.  1324.  In  order  to  provide  as  far  as  possible  against 
changes  of  temperature,  the  clock  is  inclosed  in  a  vault  of  heavy  masonry  lined  with 
tin,  situated  in  the  room  which  adjoins  the  observing-room  oil  the  eastern  side,  and  to 
prevent  the  works  of  the  clock  from  rusting  the  dryness  of  the  air  is  maintained  by 
keeping  a  tire  in  the  clock-room  throughout  the  year.  The  clock  has  a  gridiron  pen- 
dulum, and  is  connected  with  the  galvanic  system  of  the  Observatory  by  means  of  a 
platinum  point  which  projects  from  about  the  middle  of  the  pendulum-rod,  and  at  each 
vibration  touches  the  surface  of  a  small  globule  of  mercury  and  closes  tlie  circuit.  It 
is  connected  with  the  chronograph  in  the  usual  way.  The  rate  of  the  clock  is  con- 
trolled by  means  of  small  weights  which  can  be  placed  in  a  small  cup  on  the  pendulum- 
rod. 

TIIK  Cor NTI NO-CLOCK. 

This  clock  is  attached  to  a  brick  pier  near  the  eastern  wall  in  the  observing-TOOm. 
It  was  made  bv  Parkinson  &  Frodsham,  of  London,  and  has  a  mercurial  pendulum. 
Its  rate  is  controlled  by  means  of  small  weights  placed  on  the  top  of  the  jar  containing 
the  mercnrv,  and  it  is  kept  within  less  than  half  a  second  of  the  KesseN  clock. 

This  clock  contains  a  delicate  spring,  furnished  with  a  platinum  point,  roting 
upon  a  small  plate  of  the  same  metal  ;  the  \\hole  so  arranged  that  at  the  lien-inning  of 

each  minute  an  arm  attached  to  the  arbor  of  the  sec Is-hand  raises  the  spring  and 

separates  the  point  from  the  plate  for  one  second.  Wires  are  attached  to  this  apparatus 
so  that  when  desired  it  may  be  included  in  the  same  circuit  with  the  KI-- els  rlock, 
when,  if  the  heats  of  the  two  clocks  are  nearly  in  coincidence,  tin-  mark  for  the  sixtieth 
second  of  the  Kessels  clock  is  omitted  on  the  chronograph. 


24  INSTRUMENTS  OF  THE  OliSKRVATORY. 

THE  STANDARD  MEAN-TIME  CLOCK. 

This  clock  ys  attached  to  a  stone  pier  in  the  chronometer-room,  but  tin-  officer 
having  charge  of  the  Transit  Circle  is  responsible  for  furnishing  its  errors  and  rates. 
It  was  made  by  Parkinson  &  Frodsham,  of  London,  has  a  mercurial  pendulum,  and 
its  rate  is  controlled  by  means  of  small  weights  placed  on  the  jar  containing  the  mer- 
cury. 

It  is  usually  compared  with  the  standard  sidereal  clock  by  causing  their  beats  to 
be  recorded  on  the  same  chronograph,  generally,  until  a  coincidence  is  obtained,  every 
day  that  a  sufficient  number  of  observations  are  obtained  with  the  Transit  Circle  to 
determine  the  error  of  the  sidereal  clock.  The  accuracy  of  this  clock  is  far  superior 
to  that  of  any  other  clock  in  the  Observatory. 

THE  CHRONOGRAPH. 

This  apparatus  is  mounted  on  a  small  stand  near  the  eastern  wall  of  the  observing- 
room.  This  instrument  is  a  modified  form  of  the  Hipp  chronograph,  in  which  regu- 
larity of  motion  in  the  train  is  secured  by  a  spring  making  132  vibrations  per  second. 
The  barrel  of  the  chronograph  is  6.37  inches  in  diameter,  13.5  inches  long,  and  revolves 
once  in  a  minute.  A  sheet  of  paper  fastened  on  the  chronograph-barrel  by  thin  metallic 
springs  is  sufficient  to  contain  the  record  of  two  hours'  continuous  work,  and  the  train 
only  requires  winding  when  the  paper  is  changed.  The  clock-signals  and  those  of  the 
observer  are  recorded  by  a  single  pen.  The  armature  of  the  magnet  which  controls  the 
penis  provided  with  repeating-points,  for  use  in  sending  signals  to  distant  stations  when 
determining  telegraphic  differences  of  longitude. 

The  pen-carriage  is  moved  forward  by  an  endless  screw,  but  may  be  lifted  from 
its  bearings  and  moved  backward  or  forward  so  that  the  pen  can  be  placed  upon  any 
desired  part  of  the  paper.  The  chronograph  stops  when  the  sheet  is  full.  The  record 
is  made  on  paper  of  the  quality  of  ordinary  writing-paper,  witlran  ink  which  does  not 
freeze  at  o°  F.  The  ink  is  made  in' the  following  proportions: 

Water      -  4  fluid  ounces. 

Alcohol  2  fluid  ounces. 

Concentrated  Glycerine  i  fluid  drachm. 

Crystallized  Aniline  Blue  40  grains. 

The  ink  should  be  thoroughly  filtered  before  using. 

The  pen  is  of  glass,  and  the  lower  part  of  the  bulb  is  drawn  out  to  a  fine  capillary 
tube,  so  that,  when  the  bulb  is  filled  with  ink  and  the  pen  allowed  to  rest  its  whole 
weight  on  the  paper,  it  produces  a  fine,  sharp  line  as  the  barrel  revolves. 

THE  BAROMETER  AND  THERMOMETER. 

The  Barometer  is  of  the  cistern  form,  and  is  suspended  on  the  eastern  wall  of  the 
observing-room,  near  its  northeastern  corner.  This  instrument  reads  by  a  vernier  to 
the  one-hundredth  of  an  inch. 


INM  RCMICNTS  OF  THE  OBSERVATORY.  25 

Tin-  Thermometer  has  n  Fahrenheit  scale,  and  is  graduated  from  —  30 "  to  -f- 157°. 
It  is  suspended  i  tout  outside  tin-  north  front  of  the  observing-room,  13.5  feet  above 
tlie  grass,  and  is  protected  from  radiation  by  A  shield  formed  of  a  double  roof  of  boards, 

the  outside  one  covered  with   tin   painted  white,  and  three  sides  of  wooden,   double 
louver-work 

TIIK  niJSKl.'YIMJ-Unu.M. 

The  interior  of  the  obser\  ing-room,  \vhich  has  been  occupied  since  February  2, 
18/0,  measures  40  feet  from  north  to  south,  and  28  feet  3  inches  from  east  to  west. 
Its  height,  from  the  floor  to  the  under  side  of  the  shutters  which  cover  the  opening  in 
the  roof,  is  23  feet  2  indies  at  the  ridge,  and  19  feet  6  inches  at,  the  eaves.  The  ridge 
of  the  root' extends  ea>t  and  west.  The  opening  in  the  roof  has  a  width  of  3  feet  S 
inches,  and  extends  downward,  in  the  north  and  south  walls,  to  within  7  feet  4  inches 
of  the  floor.  The  portion  in  the  roof  is  closed  by  means  of  four  sliding  shutters,  while 
the.  portions  in  the  north  and  south  walls  are  each  closed  by  double  doors. 

The  construction  of  the  room  is  peculiar,  the  walls  of  the  frame  being  covered 
with  tin,  and  shaded  from  the  direct  rays  of  the  sun  on  the  east,  south,  and  west  sides 
bv  light  wooden  louver-work.  This  construction  was  adopted  in  order  to  secure 
identity  of  temperature  between  the  internal  and  external  air.  The  latitude  of  the 
Transit  Circle  is  the  same  as*  that  of  the  center  of  the  central  dome,  but  it  is  77.8  feet 
—  O9.o66  west  of  that  point. 

4 74  APP  I 


VI.-THE  XXVHNCB  EQUATORIAL. 


[Prepared  by  Professor  EDWARD  S.  HOLDEN,  U.  S.  N.] 


Section  18  of  the  "Naval  Appropriation  Bill,"  approved  July  15,  1870,  contained 
the  following'  provision : 

"That  the  Superintendent  of  the  Naval  Observatory  be,  and  he  is  hereby,  author- 
ized to  contract  for  the  construction  of  a  refracting  telescope  of  the  largest  size,  of 
American  manufacture,  at  a  cost  not  exceeding  fifty  thousand  dollars." 

Immediately  upon  the  passage  of  this  bill  negotiations  were  entered  into  with  the 
firm  of  ALVAN  CLARK  &  SONS,  of  Cambridge,  Massachusetts,  resulting  in  the  conclu- 
sion of  a  contract  for  the  construction  of  the  instrument,  in  the  following  terms : 

''This  indenture,  made  and  concluded  upon  this  thirteenth  day  of  August,  in  the  year  one  thou- 
sand eight  hundred  and  seventy,  between  ALVAN  CLARK,  GEORGE  B.  CLARK,  and  ALVAN  G. 
CLARK,  nil  of  Cambridge,  Massachusetts,  and  doing  business  under  the  name  aud  firm  of  ALVAN 
CLARK  &  SONS,  as  the  party  of  the  first  part,  aud  the  UNITED  STATES  OF  AMERICA,  repre- 
sented by  Commodore  B;  F.  SANDS,  IT.  S.  N.,  Superintendent  of  the  Naval  Observatory,  by  authority 
of  the  eighteenth  section  of  the  act  making  appropriations  for  the  naval  service  for  the  year  ending 
June  30,  1871,  approved  July  15,  1870,  as  the  party  of  the  second  part,  wituesseth  : 

"  That,  lor  the  considerations  hereinafter  mentioned,  the  said  party  of  the  first  part  agrees  to 
construct,  finish,  and  mount  in  the  United  States  Naval  Observatory,  Washington,  free  of  all 
expense  to  the  party  of  the  second  part,  a  refracting  telescope  of  good  definition,  and  of  twenty-six 
inches  clear  aperture,  mounted  equatorially  on  the  German  plan,  with  all  the  usual  and  necessary 
counterpoises  and  other  devices  to  secure  its  easy  motion,  its  proper  balance  in  all  positions,  and 
the  safety  of  its  object-glass;  with  divided  circles,  to  be  each  read  by  two  microscopes ;  the  K.  A. 
circle  to  seconds  of  time,  and  the  Declination  circle  to  tenths  of  a  minute  of  arc;  with  a  driving- 
clock  sufficiently  regular  and  powerful  to  secure  an  equable  and  steady  motion  in  right  ascension, 
for  a,  period  of  at  least  three  hours  without  stopping;  with  the  usual  number  and  description  of 
eye-pieces  and  ring-micrometers;  with  a  filar  micrometer  for  measuring  distance  and  angle  of 
position  ;  an  AIRY'S  double-image  micrometer,  and  a  divided  glass  or  mica-scale  micrometer.  The 
said  telescope  shall  also  be  furnished  with  such  spectroscope,  such  devices  to  secure  successful  and 
easy  manipulation,  and  such  apparatus  for  illuminating  the  wires,  the  field,  and  the  circle-divisions, 
as  the  party  of  the  second  part  may  prescribe,  it  being  agreed,  however,  that  no  spectroscope  shall 
be  required  by  hun  of  which  the  total  necessary  expense  of  construction  shall  exceed  one  thousand 
dollars,  and  that  no  new  devices  or  apparatus  thus  prescribed  shall  be  of  an  uuu'sually  expensive 
character. 

"  It  is  also  agreed  that  the  party  of  the  second  part  shall  build  at  his  own  expense  a  proper 
foundation  of  the  instrument,  to  include  all  that  part  of  the  support  which  is  commonly  built  of 
stone,  and  to  be  terminated  on  top  by  a  horizontal  surface,  and  that  the  contracting  party  shall 
furnish  aud  put  in  complete  working  order  everything  which  is  to  be  borne  by  the  said  surface,  and 
shall  do  everything  necessary  to  secure  the  successful  working  and  performance  of  the  instrument, 
whether  otherwise  expressed  in  this  contract  or  not. 

"  It  is  also  agreed  that  the  said  instrument  shall  be  completed  aud  mounted  within  four  years  of 
the  time  of  receiving  the  first  payment. 


INSTKI'MKNTS  ()K  T1IK  OBSERVATORY.  27 

"  It  is  also  agreed  that  all  the  designs  of  the  instrument  shall  be  submitted  to  the  Superintendent 
of  the  Naval  Observatory  and  lie  approved  by  him  before  being  carried  into  execution,  but  such 
approval  shall  not  be  construed  to  diminish  the  responsibility  of  the  contractors. 

"  In  consideration  of  the  foregoing,  the  party  of  the  second  part  agrees  to  pay  the  party  of  the 
first  part  the  sum  of  forty-six  thousand  (46,000)  dollars  in  lawful  money  of  the  United  States,  in  time 
and  in. inner  as  follows,  that  is  to  say,  a  payment  of  ten  thousand  dollars  when  the  glass  shall  have- 
been  tested  and  found  of  proper  ((Utility  :  an  additional  payment  of  live  thousand  dollars  when  the. 
labor  of  figuring  the  glass  shall  be  half  performed ;  a  payment  of  seven  thousand  live  hundred 
dollars  when  the  said  labor  shall  have  been  completed  and  the  glass  proved  in  its  cell  ;  a  payment 
of  five  thousand  dollars  when  the  instrument  shall  be  ready  for  mounting:  and  the  remainder  of 
the  price  when  the  party  of  the  first  part  shall  bave  fully  completed  their  contract.  Hut  it  is  agreed 
that  if  the  party  of  the  second  part  shall  dispense  with  the  spectroscope,  a  deduction  of  one  thou- 
sand dollars  shall  be  made  from  the  above-mentioned  price,  and,  furthermore,  that  if  the  I'n'ited 
States  shall  remit  the  import  duty  on  the  raw  material,  or  any  part  of  it,  a  deduction  shall  be  made 
from  the  price  equal  to  the  currency-value  of  the  duties  thus  remitted.  The  party  of  the  second 
part  also  reserves  the  right,  before  unking  any  payment,  to  require  the  contracting  party  to  enter 
into  a  bond,  with  approved  security,  conditioned  on  the  faithful  performance  of  the  contract,  and  to 
insure  the  work  against  injury  by  fire  at  their  own  expense." 

The  glass  disks  were  immediately  ordered  by  the  Contractors  from  Messrs.  CHAM  i: 
BHOTHEES  cc  Co.,  of  Birmingham,  England,  but  such  was  the  difficulty  of  casting  disks 

of  the  required  sixe  that  they  did  not  arrive  in  this  country  until  the  end  of  December, 
1871.  The  work  of  figuring  them  was  immediately  commenced  and  was  completed 
in  the  autumn  following.  The  mounting  of  the  telescope  was  finished  in  the  summer 
of  1873,  and  the  whole  was  brought  to  Washington  in  October  of  that  year.  The 
work  of  mounting  was  completed  during  the  month  following,  and  on  November  20 
regular  observations  were  commenced.  The  construction  of  the  telescope  and  the 
building  of  the  dome  was  under  the  immediate  direction  of  Professor  Xr.w<  OMH.  It  is 
worth  v  of  remark  that  the  instrument  was  completed  in  less  than  half  the  time  Stipulated 
in  the  contract. 

THE  DOME. 

The  erection  of  the  instrument  upon  the  Observatory  building  would  have  required 
the  removal  of  the  old  lo-inch  equatorial,  the  remodeling  of  the  interior  of  the  build- 
ing, and  the  erection  upon  it  of  a  dome  out  of  all  proportion  to  the  structure.  It 
was  therefore  decided  to  extend  the  south  wing  of  the  Observatory,  and  to  place 
the  floor  of  the  great  dome  upon  the  same  level  with  the  main  floor  of  the  rest  of  the 
building.  The  disadvantages  of  this  course  were  believed  to  be  far  more  than  coun- 
terbalanced by  its  advantages.  The  only  disadvantage  to  be  taken  into  consideration 
is,  that  the  telescope  cannot  sweep  the  horizon.  In  the  north,  the  old  dome  rises  to  an 
altitude  of  about  16°,  while  in  other  directions  the  surrounding  trees  interfere  with  the 
view  to  an  average  height  of  perhaps  10°.  This  drawback  is  however  entirely  unim- 
portant, for  the  reason  that  with  so  large  an  instrument  astronomical  observation*  can 
never  be  advantageously  made  so  near  the  horizon.  During  the  two  years'  use  of  the 
instrument  no  case  has  yet  arisen  in  which  the  view  of  any  object  which  it  was  de.-ired 
to  observe  has  been  cut  oil'  bv  tlioe  obstructions.  The  advantages  of  the  arrangement 
are  cheapness  and  simpliritv  of  construction,  convenience  of  access  from  the  adjoining 
computing-room,  and  partial  protection  from  the  wind. 

The  foundation  of  the  dome  is  rubble-stone,  43  feet  exterior  diameter,  and  2  feet 


28  INSTRUMENTS  OF  THE  OBSERVATORY. 

thickness  of  wall.  It  extends  1 7  feet  below  the  main  floor,  and  on  the  average,-  about  1 2 
feet  below  the  surface  of  the  ground.  This  stone  foundation  extends  only  to  the  bottom 
of  the  joists  which  support  the  floor.  On  it  rests  a  circular  sill  of  oak  timber  in  8-foot 
sections,  which  is  fastened  down  by  iron  anchors,  each  extending  3  feet  into  the  stone- 
work. On  this  sill  rest  vertical  posts  of  oak  timber,  alternately  6  inches  by  6  inches 
and  3  inches  by  6  inches,  which  form  the  frame-work  of  the  cylindrical  portion  of  the 
dome.  The  interior  diameter  of  this  portion  is  41  feet,  and  its  height  14  feet.  These 
joists  are  strengthened  by  six  diagonal  braces  between  each  pair  of  posts,  the  alternate 
sets  inclining  in  opposite  directions.  On  top  of  the  posts  rests  a' circular  plate  of  oak 
timber,  8  inches  broad  by  4  inches  high.  On  this  plate  rests  a  plate  of  iron,  cast  in 
pieces  about  6  feet  long,  with  flanges  5  inches  wide  on  the  edges,  which  flanges  fit 
snugly  on  the  two  vertical  sides  of  the  oak  plate,  so  that  the  latter  is  inclosed  by  iron 
surfaces  on  three  sides.  The-  inner  and  outer  edges  of  the  iron  plates  are  elevated  so 
as  to  form  a  sort  of  railway,  with  convex  surfaces,  the  rails  being  6  inches  to  7  inches 
apart  from  centers. 

On  these  rails  rest  the  rollers,  sixteen  in  number,  on  which  the  dome  turns. 
These  are  conical  in  form,  the  outer  end  being  enlarged  in  the  proportion  of  the 
distance  it  has  to  move,  and  each  is  supplied  with  a  pair  of  flanges  to  prevent  end  motion. 
These  rollers  are  connected  by  a  linked  coupling,  like  that  employed  by  Mr.  GRUBM 
in  the  dome  of  the  Dublin  observatory,  which,  at  the  same  time,  keeps  the  rollers  at 
the  proper  distance  apart,  and  maintains  their  axes  in  a  line  passing  through  the  center 
of  the  dome.  The  principal  deviation  from  Mr.  GKUBB'S  system  is,  that  his  three  nar- 
row rollers  are  combined  into  a  single  wide  one. 

On  the  rollers  rests  another  circular  iron  plate  similar  to  the  one  upon  which  they 
rest,  except  that  only  the  central  part,  about  3  inches  broad,  rests  on  the  rollers,  and 
that  on  the  inside  a  horizontal  flange,  about  3  inches  wide,  is  cast.  Sixteen  friction- 
rollers  are  supported  at  various  points  about  the  drum  of  the  dome,  which  consist 
essentially  of  horizontal  wheels,  whose  vertical  rims  are  close  to  the  vertical  flanges 
of  this  circular  plate,  and  which  project  slightly  over  its  horizontal  flange  just  de- 
scribed. Between  the  vertical  flanges  is  set  the  8-inch  by  4-inch  wooden  sill  on  which 
the  dome  is  built. 

The  dome  is  of  the  hemispherical  pattern,  the  lower  3  feet,  however,  being  as 
usual,  cylindrical.  The  opening  is  6.5  feet  in  width,  and  extends  nearly  6  feet  beyond 
the  zenith.  The  sides  are  formed  by  two  strong  arches,  into  which  all  the  smaller  ribs 
are  nailed.  To  prevent  lateral  springing  of  these  arches,  and  to  give  solidity  to  the 
junction  of  the'arches  with  the  ribs,  a  1 2-inch  by  i-inch  pine  board  is  sprung  on  the 
frame-work  under  each  arch,  through  the  whole  course  of  the  latter,  its  outer  edge 
being  flush  with  the  inner  vertical  surface  of  the  arch.  As  the  surface  of  this  board  is 
necessarily  cylindrical,  instead  of  spherical,  the  under  edges  of  the  ribs  are  notched 
away  sufficiently  to  allow  the  board  to  fit  into  them,  and  the  board  is  then  nailed  to 
each  of  them.  That  there  has  been  no  such  lateral  springing  has  been  shown  by 
careful  measures  of  the  width  of  the  slit  at  various  times.  These  arches  have,  neces- 
sarily, a  powerful  end  thrust,  tending  to  force  the  lowest  circle  of  the  moving  portion  of 
the  dome  into  an  elliptical  form.  Tin's  has  been  provided  against  by  shrinking  two 


^LIBRARY 

UNIVERSITY  OF 

CALIFORNIA.. 


James  R.  Osgood  &  Co.,  Boston. 


THE    XXVI-INCH    EQUATORIAL. 
U.  S.   NAVAL   OBSERVATORY,   WASHINGTON.        MOUNTED    1873. 

!  Focal  Length,  32  ft.  5.8  in. 
Pl^ar     An*»rtiirp.    -jf\    in. 


INSTRUMENTS  OF  THE  OHSERVATORY.  2g 

bands  of  wrought  iron,  about  15  feet  long,  on  the  outside  of  the  dome,  so  as  to  pre- 
vent such  change  of  shape. 

The  frame-work  of  the  dome  is  covered  on  the  outside  by  sheets  of  galvanized 
iron  and  on  the  inside  by  canvas.  The  canvas  on  the  inside  of  the  cylindrical  portion 
has  been  thoroughly  soaked  with  several  coats  of  .W/(///r  </l<ixs.  Kxperiment  has  shown 
that  it  will  not  blaze  when  lire  is  applied  to  it,  but  onlv  slowly  smolder  in  such  a  way 
that  some  time  would  lie  required  before  the  wooden  frame-work  could  be  ignited  even 
if  the  canvas  were  to  be  accidentally  set  on  fire.  To  provide  against  possible  accidents 
of  this  kind,  a  hose  is  kept  attached  to  a  water-pipe  supplied  from  the  main  reservoirs 
of  the  city,  through  which  a  powerful  stream  of  water  can  be  at  once  directed  to  any 
portion  of  the  interior  of  the  dome  or  of  its  adjacent  office  .and  bed-room.  This  hose  is 
kept  ill  working  order  in  one  of  the  closets  opening  directly  into  the  dome. 

r/o.s-(7.s- .- — Three  closets  are  provided,  in  one  of  which  the  Chronograph  is  placed, 
the  other  two  serving  as  receptacles  ibr  the  smaller  apparatus,  tools,  etc. 

The  Chronograph-closet  has  within  it  the  register  of  a  hot-air  furnace,  which  is 
opened  whenever  it  becomes  desirable  (in  the  day-time,  and  not  during  observations) 
to  prevent  the  xtmith/t/  of  the  pier  and  instrument,  which  occasionally  takes  place  by 
the  deposition  of  dew  from  moist  warm  air  upon  the  cold  surfaces  of  the  telescope,  etc., 
which  have  not  yet  reached  the  temperature  of  the  surrounding  air. 

The  temperatures  inside  and  outside  of  the  dome  are  usually  very  nearly  the 
same,  owing  to  its  peculiar  construction;  generally  within  5°  of  Fahrenheit's  scale. 
Ample  provision  has  been  made  for  securing  this  essential  to  the  good  performance  of 
a  large  telescope,  in  the  two  windows  and  in  the  two  wide  doors  of  the  dome,  which 
are  usually  opened  shortly  after  sunset  on  clear  days. 

The  dome  is  lighted  by  eight  gas-burners  arranged  at  nearly  equal  distances 
around  the  drum  or  cylindric  portion,  about  6  feet  above  the  floor.  There  is  also  a 
gas-pipe  led  around  the  pier,  about  2  feet  from  the  floor,  in  which  are  six  stop-cocks, 
to  which  flexible-rubber  tubes  can  be  fitted,  one  on  the  north  and  south  ends  and  two 
on  the  west  and  east  sides.  (See  Plate  IV.)  From  this  pipe  a  branch  goes  up  the  north 
end  of  the  pier  and  of  the  "harp-shaped  piece"  of  the  mounting,  terminating  in  two 
burners  near  the  north  end  of  the  polar  axis.  The  photograph  (Plate  IV)  shows  the 
arrangement  of  these  burners.1 

The  dome  is  moved  by  means  df  a  pinion  of  cast  iron,  10  inches  in  diameter, 
which  works  into  a  rack,  also  of  iron,  cast  in  sections  of  about  4  feet  in  length,  screwed 
firmly  t<>  the  inner  surface  of  the  dome.  This  pinion  is  on  the  same  horizontal  axle  as 
a  cast-iron  wheel,  3  feet  in  diameter,  in  whose  grooved  edge  an  endless  manila  rope,  i 
inch  in  diameter,  is  laid.  The  rack,  rollers,  and  friction-rollers  are  shown  in  Plate  IV. 
This  simple  device  has  proved  satisfactorv,  after  two  years'  service.  A  pull  of  about 
liftv  pounds  is  sufficient  to  start  the  dome  to  move,  and  less  than  this  will  preserve  the 
motion. 

8HUTTBB8. 

After  trial  and  experiments  upon  various  systems  of  shutters  for  closing  the  observ- 
ing-slit,  which  is  6.5  feet  wide  and  39.5  feet  long,  the  following  very  simple  device 


1  It  i*  tolic  noted  that  Plates  IV  ami  V  art-  m-i-mil  mirth  ami  Mcmlli.     I'lati;  VI,  however,  i>  not  invn-ti'il. 


3O  INSTRUMENTS  OF  THE  OBSERVATORY. 

was  chosen,  which  after  a  year's  trial  has  shown  itself  to  be  satisfactory  and 
convenient.  A  piece  of  canvas,  43  feet  long  and  9  feet  4  inches  wide,  is  used  for  tin- 
covering  ;  to  make  it  water-tight  it  is  treated  with  two  or  three  coats  of  linseed-oil. 
One  end  of  the  canvas  is  lashed  through  eyelets  to  ring-bolts  at  the  lower  extremity 
of  the  slit,  and  the  other  end  is  fastened  to  a  cross-bar  of  i  -inch  round  iron,  which 
has  a  grooved  wheel,  4  inches  in  diameter,  at  each  end.  These  grooved  wheels  rest 
upon  brass  railway-tracks,  laid  on  the  outside  of  each  of  the  main  art-lies  which 
limit  the  slit,  and  travel  up  and  down  upon  these  tracks  as  the  cross-bar  is  raised  or 
lowered.  In  the  longer  sides  of  the  canvas,  galvanized-iron  rings  are  inserted  every  3 
or  4  feet,  and  these  rings  embrace  two  arches  composed  of  stiff  galvanized-iron  pipes, 
2  inches  in  diameter,  which  are  bent  to  a  circle  of  a  radius  3  or  4  inches  greater  than 
the  outside  radius  of  the  sphere  of  the  dome.  The  ends  of  these  pipes  are  secured  by 
passing  them  through  oak  blocks  bolted  firmly  to  the  outside  of  the  dome.  To  the 
iron  cross-bar,  at  the  upper  end  of  the  canvas,  two  wire  ropes  are  attached,  each  of 
which  runs  from  its  end  of  the  cross-bar  through  a  series  of  nicely-fitted  brass  guide- 
blocks  or  fair-leaders,  eight  in  number,  fastened  to  the  faces  of  the  main  arches,  up  to 
the  outside  of  the  dome  and  to  a  cast-iron  wheel,  i  foot  in  diameter ;  returning,  the 
rope  passes  round  this  wheel  and  through  a  second  series  of  guide-blocks,  immediately 
below  the  first,  and  round  a  second  iron  wheel,  of  the  same  size  as  that  at  the  top  of 
the  slit,  at  the  end  of  a  horizontal  axle  placed  at  the  bo"ttom  of  the  slit  and  inside  of 
the  dome,  and  from  thence  back  to  the  other  side  of  the  cross-bar  to  the  point  where 
it  is  first  fastened.  It  is  thus,  practically,  an  endless  rope. 

Motion  can  be  given  to  the  horizontal  axle,  at  the  bottom  of  the  slit,  by  a  simple 
mechanical  arrangement.  This  motion  is  at  once  communicated  to  the  iron  bar  at  the 
upper  end  of  the  canvas  through  the  wire  ropes,  and  the  slit  is  opened  or  closed  with  ease. 
The  office  of  the  2-inch  galvanized-iron  arches  on  the  outside  of  the  dome  is  to  con- 
fine the  sides  of  the  canvas  by  means  of  the  rings  which  slip  upon  them,  so  as  to  pre- 
vent flapping  of  the  canvas  in  case  of  high  winds,  and  to  keep  its  edge  so  close  to  the 
dome  that  rain  may  not  beat  in  under  it.  The  railway-tracks  direct  the  motion  and 
lessen  friction.  Simple  mechanical  arrangements  are  provided  for  lengthening  or 
shortening  the  wire  ropes,  and  for  causing  the  direction  of  the  pull  of  these  ropes  to 
be  exerted  upon  the  iron  cross-bar  in  the  most  advantageous  way.  This  shutter  can 
be  opened  or  closed  in  two  minutes  by  one  man,  and  it  is  perfectly  water-tight,  the 
canvas  requiring  renewal  about  once  a  year.  In  a  more  northern  latitude  snow  might 
seriously  interfere  with  its  efficiency.  During  the  winter  of  1874-' 7 5  at  Washington, 
however,  no  trouble  was  experienced  from  this  cause. 

OBSKRVING-CHAIRS. 

The  large  observing-chair  and  step-ladder,  shown  in  Plate  IV,  is  of  the  simplest  and 
lightest  construction,  and  may  be  understood  by  an  examination  of  the  photograph. 
It  is  not  placed  upon  a  track,  as  it  serves  as  a  step-ladder  as  well  as  a  chair,  and  moves 
upon  the  solid  floor  of  the  dome  on  ordinary  casters.  The  chair  itself  is  over-counter- 
poised, and  will  move  upward  by  the  application  of  a  slight  force  and  downward  when 
the  observer  is  seated  in  it.  To  adjust  it  to  any  position  of  the  telescope,  it  is  moved 


INSTRUMENTS  OF  THE  OBSERVATORY.  3! 

slightly  above  the  proper  place,  the  observer  enters  it.  and  as  it  moves  slowlv  down- 
ward he  fixes  it  in  position  by  the  motion  ot'a  lexer  which  interposes  a  stop  in  a  toothed 
rack.  The  lower  portion  of  the  frame-work  carries  a  closet  in  which  are  placed  a 
Ruhmkorff  coil  and  a  hatterv. 

This  chair  is  not  available  for  observations  near  the  zenith.  For  these  an  ordi- 
nary ••invalid-chair"  is  provided,  which  serves  for  all  observations  from  the  zenith 
southward  to  75°  of  north  polar  distance,  at  which  point  the'  larger  chair  becomes 
available. 

THE   PIER. 

At  its  foundation  the  pier  which  supports  the  telescope  is  divided  into  four  parts, 
each  5  feet  by  3  feet,  the  longer  sides  being  north  and  south.  These  are  set  so  as  to 
leave  an  opening  14  feet  long-  from  north  to  south  and  3  feet  10  inches  wide  from  east 
to  west.  The  opening  in  the  shorter  direction  is  arched  over  at  q,  height  of  8  feet  above 
the  floor  of  the  cellar,  and  the  longer  opening,  running  north  and  south,  at  a  height  of 
i  2  feet.  The  latter  arch  is  built  of  brick,  as  is  also  the  pier  above  it.  At  the  level  of 
the  floor  of  the  dome  the  brick  pier  is  nearly  1 1  feet  long  from  north  to  south,  and  4 
feet  from  east  to  west.  It  tapers  slightly  on  each  side  to  the  top.  It  is  surmounted  by 
a  stone  cap  2  teet  by  2  feet  by  8  feet,  on  which  rests  the  cast-iron  support  ("harp- 
shaped  piece")  of  the  telescope.  Above  the  floor  of  the  dome  and  within  the  brick 
pier  is  a  closet,  whose  floor  is  covered  by  an  iron  base  (for  the  support  of  the  driving- 
clock)  about  3  feet  square,  the  inside  height  of  the  closet  being  about  5.5  feet.  This  closet 
(shown  in  Plate  IV)  is  closed  by  two  doors  on  its  east  and  west  sides,  and  the  driving- 
clock  is  thus  thoroughly  protected  from  dust,  etc.  The  cap-stone  is  secured  to  the  brick 
pier  by  four  solid  iron  rods  which  pass  completely  through  the  base  of  the-  harp-shaped 
piece  and  through  the  cap-stone,  and  are  secured  by  nuts  inside  the  brick  pier  and  above 
the  base  of  the  casting.  In  Plate  IV  the  position  of  the  southeast  rod  may  be  traced  from 
the  nut  at  the  top  of  the  base  of  the  casting  downward  to  the  hole  in  the  brick  pier,  in 
which  the  under  nut  is  secured.  This  hole  is  printed  quite  dark  in  the  photograph. 
These  four  rods  make  the  brick  pier,  the  cap-stone,  and  the  harp-shaped  piece  one 
connected  system. 

THE  MOUNTING. 

The  harp-shaped  piece,  while  it  is  strongly  connected  with  the  cap-stone  and  pier, 
is  yet  capable  of  three  motions:  one  in  azimuth;  one  a  rocking  motion  in  zenith- 
distance,  about  a  north  and  south  line;  and  one  a  rocking  motion  in  zenith-distance, 
about  an  east  and  west  line.  Before  the  harp-shaped  piece  was  mounted,  two  pieces  of 
cast  iron  were  firmly  bolted  to  the  top  of  the  cap-stone  5  feet  5  inches  apart.  These 
serve  simplv  for  four  horizontal  screws  to  abut  against,  by  whose  motion  the  whole 
instrument  is  slewed  in  azimuth.  Plate  IV  shows  two  of  these  screws,  the  head  of  tin- 
southeast  screw  being  just  below  the  handle  of  the  crank  for  rapid  motion  in  right- 
ascension,  and  the  head  of  the  northeast  screw  being  vertically  under  the  upper  bear- 
ing of  the  polar  axis.  Immediately  above  these  -crew-heads  an-  the  heads  of  two  vertical 


32  INSTRUMENTS  OF  THE  OBSERVATORY. 

screws  which,  with  two  other  similar  screws  on  the  west  side  of  the  pier,  serve  to  give 
to  the  mounting  the  two  motions  in  zenith-distance  just  described.  It  lias  been  found 
that  the  stability  of  this  mounting  leaves  little  to  be  desired. 

THE    AXES   AND    CIRCLES. 

The  polar  axis  is  7  inches  in  diameter  at  its  thickest  part,  tapering  slightly  toward  its 
lower  end.  The  north  end  of  this  axis  is  19  feet  above  the  floor  of  the  dome.  The 
declination-axis  is  of  the  same  size,  and  is  pierced  through  its  length  by  ;i  c\  lindrical 
opening,  about  2  inches  in  diameter.  A  steel  flexure-bar  passes  through  this  opening, 
one  end  of  it  being  firmly  fastened  to  a  frame-work  inside  the  tube  of  the  telescope, 
as  near  to  the  cone  of  rays  as  it  was  judged  safe  to  place  it  This  bar  attains  its 
greatest  diameter  about  a  foot  outside  the  telescope-tube,  where  it  accurately  fills  the 
bore  of  the  declination-axis.  It  then  tapers  gradually  toward  the  outer  end,  the 
amount  of  the  taper  being  so  calculated  that  the.  weight  which  the  bar  carries  at  this 
end  shall  be  somewha't  less  than  is  nec'essary  to  make  the  rod  touch  the  side  of  the 
bore. 

Friction-Hollers  and  Counterpoises: — The  north  end  of  the  polar  axis  rolls  in  a  brass 
box  and  is  supported  by  two  friction- rollers  of  steel,  about  7  inches  in  diameter. 
The  weight  of  the  axis  is  taken  from  the  box  by  means  of  a  strong  steel  spring,  several 
feet  in  length,  which  is  shown  in  Plate  IV,  just  under  the  polar  axis  and  to  the  south  of 
the  harp-shaped  piece.  The  lower  end  of  this  is  perforated,  and  traversed  by  a  bolt, 
whose  north  end  is  fixed  to  the  casting  and  whose  south  end  has  a  screw-thread  cut 
upon  it.  A  nut  on  the  screw-thread  confines  this  spring  in  any  required  position. 
For  the  declination-axis  there  are  no  friction-rollers.  The  counterpoises,  composed  of 
four  heavy  iron  cylinders,  are  so  disposed  that  the  center  of  gravity  may  fall  within 'the 
upper  box  of  the  polar  axis. 

The  Circles. — The  hour-circle  is  divided  on  its  southern  face  to  one  minute  of  time, 
and  is  read  by  two  microscopes  marked  E.  and  W.  (east  and  west)  to  one  second  of  time. 
These  microscopes  are  reached  by  means  of  wooden  steps,  permanently  attached  to 
the  south  end  of  the  pier.  There  is  also  a  coarse  division  on  the  edf/i;  <>t'  the  cifcle  in 
black  and  white  paint,  which  can  be  read  from  the  floor  by  the  unassisted  eye  to  one  . 
minute  of  time,  and'  by  means  of  this  latter  division  the  telescope  can  be  set  for  any 
object  which  can  be  recognized  in  the  finder.  The  declination-circle  is  close  to  the 
telescope-tiibe,  and  can  be  read  by  two  opposite  verniers  to  0.2  of  one  minute  of  arc. 
These  verniers  are  read  from  the  eye-end  of  the  telescope  through  two  long  micro- 
scopes, (marked  A  and  B,)  into  which  the  images  of  the  divisions  of  the  circle  and  of 
the  vernier  are  reflected  by  means  of  two  right-angled  prisms  of  glass  fixed  to  the 
tube.  The  declination-circle  is  also  divided  coarsely  on  its  edge  to  one  degree  of 
polar  distance,  and  can  be  set  by  means  of  an  opera-glass  to  one-half  of  one  degree. 
For  many  purposes  these  coarse  divisions  of  the  two  circles  are  the  only  ones  which  are 
needed. 

Plate  IV  shows  the  east  microscope  of  the  hour-circle  and  both  microscopes  of  the 
declination-circle,  as  well  as  one  prism  and  the  index  for  the  coarse  divisions.  Both 
the  fine  and  coarse  divisions  of  each  circle  can  be  read  in  the  day-time  with  ease.  At 


LIBRARY 

UNIVERSITY  OP 

CALIFORNIA.. 


Plate  V 


Heliotype. 


James  R.  Osgood  &  Co.,  Hoston. 


DETAILS  OF  CLOCK-WORK,  ETC.,  OF  XXVI-INCH  EQUATORIAL. 


INSTRTMKNTS  OK  THK  OHSKKVA  1  <  )R V. 


33 


night  the  coarse  divisions  of  both  circles  c;in  he  made  visible  by  lighting  one  or  two 
of  the  gas-burners  of  the  dome.  To  read  tlir  tine  di\  isioiis  of  the  hour-circle  oil  ha i id- 
lamps  must  be  placed  on  stands  tilted  for  this  purpose.  For  illuminating  the  fine 
divisions  of  the  declination-circle  it  was  originallv  intended  to  use  (Jeissler  tubes,  driven 
by  a  liiihmkorn"  coil,  and  this  method  was  given  a  thorough  trial.  It  was  found  that 
while  enough  light  could  be  thrown  on  the  circle,  the  management  of  the  coil  and  battery 
was  far  too  annoying,  and  the  very  simple  device  was  resorted  to  of  running  a  gas-pipe  up 
the  north  end  of  the  harp-shaped  piece,  at  the  end  of  which  two  stop-cocks  were  fitted,  to 
which  were  attached  flexible-rubber  tubes  terminating  in  gas-lamps  convenient  to  hold 
in  the  hand.  A  >et  of  steps  was  put  on  the  east  side  of  the  casting,  and  a  small  plat- 
form which  does  not  in  any  way  interfere  with  the  moving  of  the  instrument  nor  with 
its  stability,  ^as  tixed  to  its  north  end.  To  set  the  circle  the  instrument  is  brought  into 
the  meridian  and  set  by  means  of  the  coarse  divisions  to  within  less  than  one  degree  of 
north  polar  distance  of  the  required  position,  an  assistant  mounts  upon  the  platform 
and  holds  the  gas-lamp  near  the  vernier,  and  the  fine  setting  is  accomplished  by  the 
observer  >eated  in  the  observing-chair,  the  declination-clamp  and  slow-motion  screw 
being  convenient  to  his  hand. 

This  method  is  perfectly  satisfactory,  and  does  away  with  all  the  annoyance  insep- 
arable from  complicated  devices.  \  \\  \ 

y 


THE  TELESCOPE. 


. 
. — The   tul)e   is  of  steel,  ^  of  an  inch  in  thickness  near  the   ends  and  |  in 

the  middle,  and  is  made  in  three  sections.  It  is  about  32  feet  in  length,  the  middle 
one-third  being  about  31  inches  in  diameter.  From  this  middle  one-third  the  tube 
gradually  tapers  to  about  28  inches  at  the  object-end  and  21  inches  at  the  eye-end. 
This  latter  is  closed  by  a  brass  plate,  into  which  the  sliding-tubes  for  holding  the 
position-circle,  eye-pieces,  &c.,  fit.  The  photograph  (Plate  V)  gives  the  best  notion  of 
the  arrangement  of  the  eye-end.  At  the  object-end  of  the  tube  are  two  iron  doors,  about 
4  inches  square,  which  can  be  removed  for  ventilating  the  tube.  Such  experiments  as 
have  been  made  indicate  that  the  difference  of  temperature  of  the  air  outside  and  inside 
the  tube  is  usually  not  greater  than  i°  Fahrenheit.  One  of  the  ventilators  is  shown 
in  Plate  IV  close  to  the  cell  of  the  object-glass. 

Ohjri-1-ulttxs. — The  object-glass  is  composed  of  an  equi-eonve.x  front  lens  of  crown- 
glass  and  a  nearly  plano-concave  flint  lens,  the  clear  aperture  being  26  English  inches, 
and  the  principal  focal  distance  of  the  combination  being  390  inches,  nearly.  The  radii 
of  the  three  curved  surfaces  are  each  about  161  inches.  The  thickness  of  the  objective 
at  the  center  is  about  2.87  inches.  It  is  confined  in  a  gun-metal  cell  which  has 
adjusting-screws  for  collimation,  etc.  The  objective  cannot  be  rotated  in  its  cell. 

The  glasses  are  free  from  all  hurtful   rings  and   stria',  and  are  of  nearly  perfect 

figure.      To  test    their    figure,    the    telescope,    with  the    full    aperture  ot    26    inches, 

was    pointed   upon    Uranus    March    10,    1874,  and    the  eye-piece,   magnifying  600 

diameters,  \\as  carefully  focused  on  the  two  satellites  Olimn  and  Titmitn.     The  central 

5 74  APP  I 


ALIFOHNIA 

id  i  in    -  ' 


36  INSTRUMENTS  OF  THE  OBSERVATORY. 

bisections  of  the  satellite  with  the  micrometer-wire  were  made,  any  error  peculiar  to 
the  bisecting  of  the  satellite  with  the  movable  wire  would  remain.  In  practice,  however, 
the  micrometer-wire  is  moved  to  the  other  side  of  wire  A  and  set  at  the  proper  distance, 
approximately.  Wire  A  is  now  brought  by  the  motion  of  screw  A  accurately  upon 
the  satellite,  and  the  movable  wire  is  brought  to  bisect  the  planet's  disk.  The  whole 
process  of  repeated  trial  arid  rectification  is  now  gone  through  with  until  a  satisfactory 
bisection  is  attained,  and  the  reading  of  the  scale  and  micrometer-head  now  obtained, 
joined  with  that  first  read,  gives  a  measure  of  double  distance.  It  will  be  observed  • 
that  in  both  positions  of  the  micrometer-wire  relative  to  wire  A,  the  eye  is  occupied 
with  but  one  question  at  a  time  and  has  simply  to  decide  whether  a  single  object  is 
accurately  bisected  by  a  certain  wire  or  not.  It  is  not  required  that  the  eye  should 
decide  this  while  the  hand  is  employed  in  altering  the-distance,  but,  the  wires  remaining 
at  a  given  distance  apart,  the  eye  has  simply  to  decide  whether  the  given  distance  is 
the  one  required,  and  the  final  process  is  identically  the  same  for  both  positions  of  the 
micrometer-wire.  It  is  conceived  that  in  this  way  the  likelihood  of  constant  errors  is 
lessened.  It  should  be  noted  that  the  final  movement  of  the  screw  M  is  always  made 
so  that  this  screw  acts  against  the  springs  of  the  micrometer. 

The  value  of  one  revolution  of  the  screw  of  this  micrometer  has  been  carefully 
determined,  from  804  transits  of  equatorial  and  zenith  stars  on  eleven  different  nights, 
to  be 

9".948o  ±  o".ooi5. 

Owing  to  the  difficulty  of  getting  a  convenient  bright  field-illumination  at  the  time 
these  measures  were  made  all  the  transits  were  observed  over  bright  wires  in  a 
dark  field,  and  the  coincidences  of  the  fixed  and  movable  wires  were  observed  in  the 
same  way.  This  accounts  for  the  large  probable  error  which,  from  so  many  tran- 
sits, observed  Avith  a  telescope  of  so  great  focal  length  with  the  advantage  of  the 
greater  precision  of  black  wires  in  a  bright  field,  would  be  smaller.  Professor  NEW- 
COMB  has  shown  (Washington  Astronomical  Observations,  1873,  p.  civ)  that  the 
value  of  one  revohition  of  this  screw  is  sensibly  uniform  throughout,  and  that  the 
periodic  inequalities  are  inappreciable.  This  micrometer  has  been  used  from  the 
mounting  of  the  instrament,  in  November,  1873,  to  the  present  time.  The  illumina- 
tion of  the  wires  has  been  generally  accomplished  by  the  aid  of  a  hand-lamp  held 
by  an  assistant,  and  this  method  has  been  found  to  be  very  convenient  and  satis- 
factory for  the  class  of  work  hitherto  done.  Although  means  have  been  provided  to  illu- 
minate the  field  of  view,  all  observations  have  hitherto  been  made  in  a  dark  field  with 
illuminated  wires.  From  measures  by  Professor  HALL,  the  thickness  of  the  unillu- 
mihated  wires  projected  against  the  bright  disk  of  Jupiter  has  been  found  to  be  o".2^i. 
The  second  position-filar-micrometer  has  for  its  primary  object  the  measures  of 
differences  of  right  ascension  and  north  polar  distance.  For  this  purpose  the  reticle 
is  arranged  with  three  fixed  wires  for  determination  of  differences  of  right  ascension, 
and  the  micrometer-screw  carries  a  system  of  five  wires  for  measures  of  differences  of 
north  polar  distance.  The  inconvenience  of  reading  the  whole  revolutions  from  a 
small  scale  on  the  edge  of  the  micrometer-box  is  obviated  by  causing  the  micrometer- 


INSTRUMENTS  OF  THE  OBSERVAToKY 


37 


screw  M  to  carry  two  divided  heads  whose  planes  arc  parallel  to  each  other  and  close 
together.  On  one  of  these  heads  the  decimals  of  a  revolution  are  counted,  and  on  the 
adjacent  one  whole  revolutions  are  registered.  Both  heads  are  read  by  the  same  index 
or  pointer,  which  is  placed  between  the  two.  Plate  V  shows  this  micrometer  detached 
from  the  telescope,  and  also  the  spectroscope  and  double-image  micrometers,  which 
have  the  same  device. 

A  thermometer  has  been  imbedded  in  the  micrometer-box  itself,  so  as  to  be  in 
metallic  contact  with  the  micrometer-screw.  From  this  thermometer  the  true  thermal 
condition  of  the  whole  micrometer-box  can  be  ascertained,  and  as  the  illumination  of 
the  wires  is  intended  to  be  done  by  means  of  Geissler  tnbes  driven  by  a  Ruhmkorff 
coil,  thus  avoiding  the  heat  from  oil  or  gas  lamps,  it  is  hoped  that  an  accurate  deter- 
mination of  the  value  of  the  temperature  co-efficient  of  this  screw  can  be  obtained. 
The  micrometer-screws  of  both  the  filar  micrometers  were  cut  on  the  same  tool;  the 
lirst  (Micrometer  1)  has  53.265  turns  to  the  inch,  the  second  (Micrometer  II)  has 
53-318. 

A  new  double-image  micrometer,  also  by  CLAKK  &  SONS,  has  been   furnished 
which  has  the  advantage  of  a  larger  field  of  view  than  the  ordinary  instruments  of  this 
construction. 

/.///-///Vrr.s. — There  are  four  negative  eye-pieces,  of  the  AIRY  pattern,  which  fit 
into  the  slipping  tube  of  the  telescope  when  the  micrometer  is  removed.  Their  magni- 
fying powers  and  fields  of  view  are  as  below. 

NEGATIVE   EYE-PIECKS. 


Designation. 

Magnifying 

Field  of 

Makers. 

power. 

view. 

Diameters. 

/ 

A, 

'55" 

25.2 

Clark  &  Sons. 

A,, 

439" 

10.5 

Do. 

A,,, 

863' 

3-5 

Do. 

Ann 

1,360" 

2-5 

Do. 

•  Approximate. 


A  BRADLEY  rhomboidal  micrometer  and  several  ruled-glass  scales  of  various  pat- 
terns have  been  ruled  for  use  with  the  telescope  by  .Mr.  WILLIAM  A.  ROGERS,  assistant 
in  the  Harvard  College  observatory. 


INSTRUMENTS  OF  THE  OBSERVATORY. 


The  micrometer  eye-pieces  (which  fit  either  of  the  two  filar-micrometers)  are  as 
below : — 

POSITIVE    EYE-PIECES. 


Designation. 

Magnifying. 
power. 

Field  of 
view. 

Makers. 

Remarks. 

I 

Diameters. 
173 

/ 

17.  2 

Clark  &  Sons    . 

2 

284 

8  i 

Kahler   . 

•5 

^Q2 

Q   O 

Clark  &  Sons 

3  A 

AGO* 

'  6.  6 

Kahler 

4 

6*6 

i.  $ 

Clark  &  Sons  

5  A 

606 

4..  2 

Kahler  

Achromatic. 

6A 

888 

2.  3 

Steinheil  .... 

Do. 

7  C 

761 

1.  2 

Kahler  

8F 

87C 

•a.  2 

do       

Flint-glass. 

I.  IO"? 

2    6 

do 

lo 

1,282 

2.  I 

do  

II 

I,  802 

1.6 

....do  

I 

-3QO 

4.  O 

....do  

Single  lens. 

II 
III 

585 

780 

3-2 
2    6 

....do  
do  

Do. 

Do. 

IV 

i  c6o 

i  8 

do 

Do. 

*  Approximate. 

Finders. — There  are  two  finders,  one,  by  CLARK  &  SONS,  5  inches  in  aperture, 
72.5  inches  focal  length,  with  two  eye-pieces.  The  one  usually  used  has  a  field  of 
view  of  i  °  1 8',  and  a  magnifying  power  of  about  30  diameters.  The  other  has  a 
magnifying  power  of  about  75  diameters.  The  second  finder,  by  KAHLER,  is  situated 
90°  in  position-angle  from  the  first,  and  has  an  aperture  of  2%  inches,  a  focal  length 
of  2 1  inches,  and  carries  an  eye-piece  magnifying  about  1 2  diameters,  and  having  a 
field  of  view  of  over  3°.  This  finder  is  quite  convenient  for  use  when  the  larger  one 
is  either  too  high  or  too  low  for  ready  access.  Both  these  instruments  are  very  care- 
fully made.  Plate  IV  shows  the  small  finder  above  the  tube,  the  5-inch  finder  being  on 
the  west  side,  and  not  shown.  Plate  V  shows  the  eye  end  of  the  5-inch  finder  and 
the  whole  of  the  small  finder. 

Spectroscope. — The  instrument  is  provided  with  a  star  spectroscope  of  two  60° 
prisms,  made,  after  designs  by  Professor  NEWCOMB,  by  CLARK  &  SONS.  This  is  shown 
in  Plate  V.  It  has  three  eye-pieces. 

Dynumeter. — A  Ramsden  dynameter  has  been  made  for  use  with  the  telescope  by 
Mr.  EDWARD  KAHLER. 

Clock. — A  siderial  clock  is  mounted  on  the  east  side  of  the  dome,  and  connected 
electrically  with  the  chronograph. 

MOTIONS    OF   THE   TELESCOPE. 

The  telescope  being  mounted  on  the  German  plan,  has  motion  about  two  axes 
perpendicular  to  each  other.  Arrangements  are  provided  for  securing — 

(a)  A  slow  motion  in  North  Polar  Distance. 

(fe)  A  rapid  motion  in  Right  Ascension. 

(c)  A  slow  motion  in  Right  Ascension. 
(«.)  The  clamping  in  north  polar  distance  is  done  by  means  of  a  long,  stiff  brass 


INSTRUMENTS  OF  THE  OBSERVATORY.  30 

rod,  (shown  just  below  the  telescope-tub. •  in  I 'hit..-  IV,  but  hidden  in  Plate  V.)  The 
slowmotion  in  north  polar  distance  is  given  by  means  of  an  endless  rope  passing  round 
the  head  of  a  large  wheel  at  the  end  of  the  slow-motion  screw.  The  whole  of  this 
rope  is  shown  in  Plate  IV,  extending  from  near  the  eye-piece  to  the  wheel. 

(b.)  The  device  for  securing  rapid  motion  of  the  telescope  in  right  ascension  is 
not  completely  shown  in  Plate  IV,  but  most  of  the  important  portions  can  be  seen.     It 
consists  essentially  in  a  large  cast-iron   toothed  wheel,  having  96  teeth,  (one  tooth  to 
each  fifteen  minutes  of  right  ascension,)  which  is  damped  firmly  to  the  polar  axis  just 
above  the  hour-circle.     Immediately  below  this  wheel  Plate  IV  shows  a  small  iron  pinion 
which  turns  on  an  axis  parallel  to  the  polar  axis  of  the  instrument.     By  the  motion  of 
a  lever  on  the  west  side  of  the  pier,  (not  shown  in  the  photograph,)  this  pinion  may 
be  raised  or  lowered  so  as  to  be  in  or  out  of  gear  with  the  first-described  wheel.     This 
pinion  is  connected  by  a  small  train  to  a  horizontal  axle  running  east  and  west,  which  is 
fastened  to  the  south  end  of  the  harp-shaped  piece.     The  east  end  of  this  axle  carries 
a  crank,  (shown  in  Plate  IV,)  and  the  west  end  carries  a  wheel,  round  whose  rim  an 
endless  rope  passes,  which  reaches  nearly  to  the  floor  on  the  west  side  of  the  pier. 
If  the  observer  wishes  to  set  the  hour-circle  while  he  is  at  the  microscopes  of  this 
circle,  ( i.  <'.,  standing  on  the  steps  at  the  south  end  of  the  pier,)  he  first  depresses  the 
lever  which  puts  the  pinion  in  gear,  and  then  by  turning  the  crank  shown  in  the  pho- 
tograph he  causes  the  telescope  to  move  in  hour-angle  at  the  rate  of  15  minutes  of 
time  for  each  revolution  of  the  crank.     After  this  setting  is  completed  the  lever  must 
be  raised  and  the  pinion  tin-own  out  of  gear,  in  order  that -the  driving-clock  may  act. 
More  usually  the  rapid  motion  in  right  ascension  is  required  in  setting  the  telescope 
from  the  floor  of  the  dome  on  the  west  side  of  the  pier.     The  observer  can  depress  or 
raise  the  lever  for  throwing  the  pinion  in  or  out  of  gear  from  the  floor  by  means  of  two 
ropes,  and  the  motion  in  right  ascension  is  effected  by  a  pull  upon  the  rope  encircling  the 
rim  of  the  small  wheel-winch  is  at  the  west  end  of  the  same  axis  which  carries  the  crank. 
This  whole  apparatus  is  simple  and,  at  the  same  time,  effective ;    by  means  of 
it  this  telescope  can  be  reversed  nearly  as  quickly  as  the  lo-inch  Munich  telescope  of 
the  ( )bservatory.     It  should  be  noticed  that  to  each  end  of  the  telescope-tube  a  stout 
cotton  braided  line  is  attached,  (which  is  shown  in  the  photograph,  Plate  IV,)  for  the 
purpose  of  aiding  in  maneuvering  the  telescope.     Some  portion  of  this  is  always  within 
reach  from  the  floor  of  the  dome. 

(c.)  The  clamping  in  right  ascension  is  done  by  means  of  a  rope  on  the  northwest 
corner  of  the  pier,  which  passes  round  the  head  of  a  wheel,  shown  in  1  'late  I  V,  bet  \\  een 
the  polar  axis  and  the  telescope  and  close  to  the  large  wheel  for  rapid  motion  in  right 
a-rension.  The  slow  motion  in  right  ascension  is  secured  by  turning  a  wheel,  shown 
in  the  photograph  directly  in  the  middle  of  the  harp-shaped  piece,  and  on  its  east 
side.  The  endless  rope  for  giving  this  motion  is  shown  at  the  northeast  corner  of  the  pier. 

THE    DRIVING-CLOCK. 

The  form  of  driving-clock  adopted  is  the  invention  of  Professor  XEWCOMM,  and 
was  devised  by  him  with  special  reference  to  the  xxvi-inch  equatorial. 

It  will  be  convenient  to  describe  first  that  portion  which  is  for  the  purpose  of 
giving  the  requisite  motion  to  the  train,  and  second  the  portion  whose  oflice  it  is  to 


4O  INSTRUMENTS  OF  THE  OBSERVATORY. 

regulate  the  motion  and  to  make  it  sensibly  uniform.  The  driving  part  is  mainly  situ- 
ated in  the  cellar  beneath  the  dome.  It  consists,  first,  of  a  small  reaction  water- 
wheel,  (the  "BARKER'S  Mill,")  which  is  moved  by  water  from  the  city  water-pipes. 
This  water  may  be  turned  on  from  the  dome  by  means  of  a  vertical  rod  at  the  north 
end  of  the  pier.  (See  Plate  IV.)  When  the  water  is  so  turned  on  the  reaction- 
wheel  begins  to  revolve,  and  causes  a  weight  in  the  cellar  to  ascend.  When  this 
weight  has  reached  a  certain  point  the  water-supply  is  automatically  shut  off,  and  the 
reaction-wheel  wholly  or  partially  ceases  to  turn,  while  the  weight  falls.  While  the 
first  weight  is  falling  a  weight  of  about  one-fourth  or  one-fifth  the  weight  of  the  first  is 
forced  to  ascend,  and  just  before  the  first  weight  reaches  its  kwest  point  the  water-supply 
is  again  automatically  turned  on  and  the  first  weight  is  again  raised.  The  connection 
between  these  weights,  by  which  one  is  made  to  rise  when  the  other  falls,  and  by  which 
both  weights  are  connected  with  the  driving-clock  and  the  vertical  axis  of  the  water- 
wheel,  is  that  of  the  well-known  HUYGHENS'  loop.1  The  cord  which  forms  this  loop 
is  endless  and  passes  around  four  wheels,  two  horizontal  and  two  vertical,  small  guide- 
piilleys  being  introduced  to  change  direction.  It  first  passes  round  a  horizontal  wheel 
on  the  vertical  shaft  of  the  water-wheel,  from  thence  round  a  vertical  wheel  above  and 
attached  to  the  heavier  driving-weight,  thence  round  a  horizontal  wheel  which  is  fixed 
to  the  vertical  shaft  of  the  driving-clock,  (in  the  closet  of  the  pier  above  the  floor  of 
the  dome,)  and  from  thence  to  the  vertical  wheel  of  the  lighter  weight,  and  finally 
back  to  the  point  of  beginning.2  By  close  inspection  a  portion  of  this  rope  may  be 
seen  in  Plate  V  passing  round  the  bottom  of  the  vertical  shaft  of  the  clock  just  below 
the  first  toothed  wheel  of  the  train.  In  this  way  the  driving-weight  gives  approximately 
uniform  motion  to  the  first  wheel  of  the  train,  and  from  this  first  wheel  the  motion  is 
transmitted  through  the  wheels  of  the  train  to  a  vertical  rod  on  the  west  side  of  the 
pier,  at  the  top  of  which  is  the  beveled  wheel  through  which  the  motion  is  transmitted 
to  the  sector,  etc.  This  rod  is  shown  in  Plate  V  inside  the  closet  of  the  driving-clock. 
The  first  wheel  of  the  train  is  on  the  vertical  axis  of  the  conical  pendulum,  by 
means  of  which  the  regulation  of  the  motion  is  obtained.  On  this  same  vertical  axis 
is  the  horizontal  wheel  round  which  a  portion  of  the  HUYGHENS'  loop  is  wound,  and 
also  a  heavy  horizontal  fly-wheel.  The  fly-wheel,  the  vertical  shaft  of  the  pendulum, 
the  three  inclined  supports  of  this  shaft,  and  the  pendulum  itself  are  well  shown  in 
Plate  V.  The  pendulum  is  of  the  conical  description  and  makes  one  revolution  in  two 
seconds  of  time.  To  insure  isochronism,  the  horizontal  axis  of  the  fork  which  supports  the 
upper  end  of  the  pendulum  does  not  pass  through  the  center  of  the  vertical  shaft,  but  be- 
yond it,  at  such  a  distance  that  the  average  inclination  of  the  pendiilum- rod  corresponds  to 
the  minimum  velocity  of  the  axis.  The  pendulum  is  constrained  to  move  nearly  in  a  circle, 
but  it  has  a  slight  play  in  a  direction  radial  to  the  horizontal  fly-wheel,  and  this  play 
takes  place  in  a  small  narrow  slot,  about  three-quarters  of  an  inch  in  length,  inside  of 
which  the  narrow  lower  end  of  the  pendulum  moves.  The  pendulum  in  moving 
outward  in  this  slot  pushes  before  it  an  insiilated  strip  of  platinum  to  which  a 


1  HUYGHENS'  Horoloyhim  oscillatorium,  etc.,  Paris,  1673. 

8  About  95  foot-pounds  per  minute  is  required  to  drive  the  clock  in  its  present  form,  but  by  far  the  largest  part  of  this 
is  expended  in  overcoming  friction  in  the  apparatus  beneath  the  floor  of  tho  dome. 


INSTRUMENTS  OK  THE  OBSERVATORY.  4! 

copper  wire  is  attached,  which  is  connected  with  one  pole  of  an  electro-magnet. 
The  wire  from  the  other  pole  of  this  magnet  is  connected,  through  contact  springs, 
etc.,  with  the  pendulum  itself,  so  that  so  long  us  the  pendulum  touches  tin-  plati- 
num strip,  i.  c.,  so  long  us  the  clock  is  running  too  fast,  the  electro-magnet  is  in 
action,  and  attracts  its  armature  to  itself.  To  the  armature  is  attached  a  small  brake 
or  friction-pail  which  plays  against  tin-  rim  of  the  fly-wheel,  and  so  long  as  the  electro- 
magnet is  in  action  the  friction-pad  is  pressed  against  the  rim  of  the  fly-wheel,  thus 
retarding  the  motion.  The  moment  the  pendulum  drops  within  its  proper  limit  the 
brake  is  released  from  action.  The  brake  is  set  in  action  several  times  for  each  revo- 
lution when  the  clock  is  running  well.  By  constant  care  this  form  of  clock-work 
mav  be  made  to  perform  extremely  well,  but  neglect  of  oiling  for  a  few  days  causes 
irregularity  of. going.  This  constant  care  is  given  to  it  and  it  has  usually  given  good 
satisfaction. 

Tin'  Si'i'lor. — The  general  method  of  transmitting  the  motion  of  the  clock-work 
to  the  polar  axis  through  the  sector  is  similar  to  that  usually  employed  by  the  CLARKS. 
The  sector  will  run  for  more  than  two  hours  continuously,  and  a  means  is  provided  for 
quickly  setting  it  back  to  its  first  position. 

CHRONOGRAPH. 

The  Chronograph  is  peculiar  only  in  the  method  of  regulation,  which  is  a  simple 
modification  of  the  system  adopted  in  the  driving-clock  Its  conical  pendulum 
makes  one  revolution  in  one  second,  and  it  is  suspended  on  its  vertical  axis  just  as 
the  conical  pendulum  of  the  driving-clock  of  the  Equatorial.  To  insure  accurate 
circularity  in  its  path,  the  following  simple  device  is  used:  on  the  vertical  axis  of  the 
pendulum,  and  below  this,  is  placed  a  ring  with  three  arms.  This  ring  slips  around  on 
the  axis,  a  very  slight  force  serving  to  move  it.  The  three  arms  are  horizontal,  and 
at  an  angular  distance  of  120°  apart.  The  outer  end  of  each  of  these  light  arms  is 
bent  upward,  so  that,  seen  sidewise,  the  arm  looks  somewhat  like  an  ordinary  h'sh-hook, 
with  the  point  turned  in  toward  the  pendulum-axis.  The  length  of  these  arms  is  so 
regulated  that  when  the  pendulum  is  moving  in  its  proper  circular  orbit  the  lower  end 
of  it  just  swings  inside  of  the  points  of  the  arms.  If,  from  any  cause,  the  pendulum 
begins  to  move  in  an  ellipse,  (and  hence  not  uniformly,)  it  cannot  move  more  than 
120°  (that  is,  not  longer  than  one-third  of  a  second)  without  touching  one  of  the  three 
arms  as  it  swings  outward.  This  arm  will  now  be  canned  round  in  front  of  the  pendu- 
lum (carrying  the  ring  and  other  arms  with  it)  until  the  pendulum  drops  inside  the 
arm,  and  the  whole  process  is  repeated.  For  a  fixed  Observatory  this  form  of  Chrono- 
graph is  perhaps  as  convenient  as  any  other,  and  the  regularity  of  the  motion  of  the 
barrel  leaves  nothing  to  be  desired. 

OBSERVATIONS  MADE  WITH  THE  XXVI-INCH  TELESCOPE. 

It  may  not  be  out  of  place  to  summarize  the  principal  work  done  with   the 
xxvi-inch   Telescope  since  its   mounting  in  November,    1873.     The    instrument    was 
in  charge  of  Professor  S.  NEWCOMB,  with  Professor  HOLDKX  as  assistant,  from  Novem- 
ber,   1873,  till  June  1 6,   1875,  when   Professor  HALL  relieved  Professor  NEWCOMB. 
6 74  APP  I 


42  INSTRUMENTS  OF  THE  OBSERVATORY. 

NEPTUNE. 

On  November  20,  1873,  the  first  measure  of  the  satellite  of  Neptune  was  made, 
and  these  measures  have  been  continued  until  the  present  time,  April  20,  1876,  when- 
ever Neptune  was  suitably  situated  for  observation. 

In  1873  measures  were  made  on  12  nights. 

1874  do.  51      do. 

1875  do.  42     do. 

1876  do.  9     do. 


In  all  1 14  nights. 

• 

These  observations  may  be  found  in  the  annual  volumes  of  the  United  States 
Naval  Observatory,  and  also  in  Monthly  Notices  Eoyal  Astronomical  Society,  volume 
xxxv,  page  49,  from  November  20,  1873,  to  February  10,  1874;  and  in  Astronomische 
Nachrichten,  No.  2061,  Band  86,  col.  321,  from  January  4  to  February  17,  1875. 

URANUS. 

From  January  8,  1874,  all  four  satellites  of  Uranus  have  been  observed  when- 
ever practicable. 

Oberon  was  observed  in     1874  on  27  nights. 

1875  on  34      do. 

1876  on  19      do. 

In  all  80  nights. 

Titania  was  observed  in     1874  on  28  nights. 

1875  on  32      do. 

1876  on  22      do. 

In  all  82  nights. 

UiribrifH  was  observed  in   1874  on  9  nights. 

1875  on  3      do. 

1876  on  8      do. 

In  all  20  nights. 

Ariel  was  observed  in     1874  on  5  nights.    • 

1875  on  4      do. 

1876  on  ii      do. 

In  all  20  nights. 


INSTRUMENTS  OF  THE  OBSERVATORY. 


43 


Those  observations  arc  to  be  found  in  the  annual  volumes  of  the  Naval  Observa- 
tory, as  well  as  in  Mmiflili/  AV/Vr.s  .l{ni/n/  A*tron<»nir<il  Soci/'t//,  volume  xxxv,  page  49, 
from  -lauuarv  S,  1874,10  .\lav  26,  1874;  and  in  AstrottOHMSche  \<irl/ficlitrn,  Xo.  2061, 
Band  S6,  eol.  321,  from  January  5  to  May  28,  1875. 

All  of  the  preceding  observations  of  the  satellites  of  both  Uranus  and  Neptune, 
which  were  made  by  1'rofessor  NKWCOMH,  have  been  discussed  by  him  in  Appendix  I 
to  the  Washington  Astronomical  Observations  for  1873. 

SATURN. 

The  satellites  of  Saturn  have  been  the  subject  of  observation  since  August  29, 
1874. 

Mini  a*  was  observed  in     1874  on  3  nights. 

RtiriJritlitN  was  observed  in  1874  on  13   nights. 

1875  on  17      do. 

• 

In  all  30  nights. 

Tctlnjs  was  observed  in     1874  on  18  nights. 

1875  on  29      do. 

In  all  47  nights. 

I) /one  was  observed  in     1874  on  18  nights. 

1875  on  31      do. 

In  all  49  nights. 

Illn'/i  was  observed  in       1874  on  19  nights. 

1875  on  31      do. 

f 

In  all  50  nights. 

Tltini   was  observed  in      1874  on  21    ni»-lits. 

1875  on  42      do. 

In  all  63  niglits. 

llill>i'rion  was  observed  in  1874  on  3  nights. 

1875  on  41      do. 

In  all  44  nights. 

Jap<-1tiK  was  observed  in    1874  on  n   nights. 

1875  on  69      do. 

In  all  80  nights. 


44  INSTRUMENTS  OF  THE  OBSERVATORY. 

These  observations  may  be  found  in  detail  in  the  Observatory  volumes,  and  in 
Monthly  Notices  Royal  Astronomical  Society,  volume  xxxv,  page  327,  for  1874,  and  in 
Astronomische  Nachrichten,  Band  87,  col.  177,  for  1875. 

A  few  measures  of  the  ball  and  rings  have  been  made,  but  the  circumstances  have 
not  been  favorable.  A  drawing  of  Saturn,  as  it  appeared  in  September,  1875,  has  been 
made  by  M.  L.  TROUVELOT,  and  a  reduced  heliotype  of  the  original  pastel  is  given  in 
Plate  VI,  Fig.  i.  Of  this  and  the  succeeding  figures  it  may,  in  general,  be  said  that 
nothing  is  laid  down  which  was  not  seen  by  more  than  one  observer.  The  exception 
to  this  is  in  the  case  of  the  notches,  represented  on  the  inside  of  the  outer  ring  of 
Saturn,  which  were  seen  by  M.  TROUVELOT  with  the  1 5-inch  telescope  of  Harvard 
College  observatory,  and  again  in  Washington,  and  of  whose  existence  he  has  no 
doubt.1 

JUPITER. 

A  few  eclipses  of  the  first  satellite  have  been  observed  as  follows : 

In  1874  4  disappearances;      4  reappearances. 

1875  4  do.  5  do. 

In  all  8  disappearances ;      9  reappearances. 

Eleven  crayon  drawings,  in  color,  of  the  appearance  of  the  markings  of  Jupiter's 
disk,  have  been  made  on  the  following  dates:  June  9,  10,  16,  18,  21,  23,  24,  Juij  8, 
13,  14,  1 6.  These  are  described  in  Monthly  Notices  Royal  Astronomical  Society,  volume 
xxxvi,  page  13,  November,  1875. 

MARS. 

Five  crayon  drawings  of  Mars  have  been  made,  on  June  14,  16,  21,  23,  and 
August  5.  These  are  described  in  the  paper  last  cited. 

DOUBLE    STARS. 

The  principal  use  to  which  the  telescope  has  been  applied  has  been  in  the  study 
of  the  satellite  systems  of  the  outer  planets,  but  on  many  occasions  close  double  stars 
have  been  observed.  The  only  system  which  has  been  continuously  followed  is  that 
of  Sirius. 

Measures  were  made  of  the  companion  of  Sirius — 

In   1873  on  i  night. 

1874  on  12  nights. 

1875  on  I0     do. 

1876  on  14     do. 

In  all  -  37  nights. 

1  See  Proceedings  American  Academy  of  Arts  and  Sciences,  1875,  page  174. 


INSTRUMENTS  OF  THE  OBSERVATORY. 


45 


The  observations  of  the  companion  of  Sirius,  during  1873,  1874,  and  1875,  may 
be  found  in  Astronomische  Nachrichten,  No.  2061,  Band  86,  col.  321,  and  in  the  annual 
volumes  of  the  Observatory. 

NEBULA. 

A  study  has  been  made  of  the  Omega  Nebula,  (HERSCHEL'S  General  Catalogue, 
No.  4403,)  and  a  pastel  drawing  of  this  on  a  large  scale  has  been  completed  by  M.  I, 
TROUVKLOT.  A  reduced  heliotype  of  this  is  given  in  Plate  VI,  Fig.  3.  This  drawing 
is  described  in  Amerir/m  Journal  of  Science  for  May,  1876,  page  341.  The  Annular 
Nebula  in  fy/ra  (G.  C.,  4447)  has  also  been  studied,  and  a  pastel  drawing  of  this  is 
reproduced  in  Plate  VI,  Fig.  2.  The  observations  upon  which  this  depends  are  given 
in  Moiitlihi  .Vo//Vrx  Jlii//nl  .\*/i-<»i<>i>tiriil  Society,  volume  xxxvi,  page  61,  December,  1875 
Preliminary  studies  have  also  been  made  in  1874,  1875,  and  1876  of  the  nebula  of 
Orion,  (Gr.  C.,  1179,)  and  an  uncompleted  sketch  of  its  central  portion  by  M.  THOII- 
VELOT  is  given  in  Plate  VI,  Fig.  4.  TKMI-KI/S  nebula  in  the  Pleiades  (G.  C.,  768)  lias 
also  been  viewed  occasionally,  as  well  as  a  few  others,  notably  the  Trifid  nebula, 
(G.  C.,  4355,)  of  which  several  sketches  have  been  begun. 

None  of  the  figures  in  Plate  VI  are  satisfactory  representations  of  the  drawings 
from  which  they  were  copied. 

7 74  APP  I 


s. 


Plate  VI. 


^LIBRARY"" 

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Fig.  i.    SATURN. 


Heliolyp*. 


N. 


Fig.  3.     OMEGA  NKBUI.A. 


James.  R.  Otgood  &  Co.,  Boslon. 

Fig.  2.    ANNUI  AR  NEBULA. 


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